Between July 1, 2023 and March 31, 2024, Buffett green-lit the purchase of nearly 26 million shares of Chubb, which was worth about $6.7 billion as of the end of March 2024. In the two trading days after this position was unmasked in mid-May 2024, Chubb stock rallied by more than $21 per share.

Here's everything we know so far about Warren Buffett's mystery stock

Although Warren Buffett is being allowed to build up his position in secrecy, select filings from Berkshire Hathaway can help narrow down which stock(s) its billionaire chief is buying.

For example, when Buffett has previously requested the confidential treatment tag, it's been for a sizable purchase (often a $4 billion to $8 billion stake). Considering that Buffett's company would absolutely be required to file a Form 4 with the SEC if its stake topped 10% (this figure is often 5% for beneficial owners), we know it has to be a large business. While a relatively inexpensive, brand-name company like Stanley Black & Decker might speak to Warren Buffett's knack for understanding consumer behaviors, it's only a $10 billion company.

The company the Oracle of Omaha is secretly investing in likely has a market cap of $50 billion or considerably larger. This would allow Berkshire to invest billions of dollars without surpassing a roughly 5% to 10% stake.

In addition to knowing that Buffett is buying shares of a big company, Berkshire Hathaway's first-quarter operating results and 13F help us pinpoint which area of focus this company can be found.

Each quarter, Berkshire Hathaway's operating results provide a comparative breakdown of its cost basis, net unrealized gains, and fair value for its three investment areas of focus. By examining Berkshire's cost bases in these three areas, and comparing to the company's buying and selling activity via its 13F, we can put two and two together to figure out where Warren Buffett is doing his buying.

In the "Banks, insurance and finance" category, Berkshire's cost basis fell from $15.71 billion in the fourth quarter (Q4 2024) to $14.27 billion in the March-ended quarter (Q1 2025). This drop is explained by the ongoing selling activity in Bank of America , as well as the disposition of Berkshire's stake in Citigroup .

The second category, "Consumer products," showed a cost basis increase from $12.66 billion in Q4 2024 to $13.76 billion in Q1 2025. This can be entirely explained by Buffett's aggressive purchase of spirits stock Constellation Brands .

The third category is "Commercial, industrial and other," whose cost basis jumped from $47.14 billion in Q4 2024 to $49.1 billion in Q1 2025. With only a few hundred million dollars in purchases for this broad-based category via Berkshire's 13F, it's clear that Buffett's mystery stock resides in this category.

Lastly, we also know it's not any of Berkshire's existing holdings, since any buying activity would have shown up in the 13F or via Form 4 filings with the SEC.

Here's what we're still left to guess about

At the same time, there's plenty we can make logical guesses about, but don't know with any certainty.

Arguably the biggest issue is that the "Commercial, industrial and other" segment comprises a boatload of potential sectors, including technology, healthcare, energy, and industrials. Almost 280 publicly traded companies have a market cap of at least $50 billion, and 136 of these companies can be traced back to the tech, healthcare, energy, and industrial sectors. That's a lot of dart-throwing!

Historically, we know that Buffett isn't a huge fan of healthcare stocks, mainly because of the time and attention needed to keep up on clinical studies and Food and Drug Administration approvals.

Something similar can be said about technology stocks, whose innovations aren't often fully understood by the Oracle of Omaha. For instance, even though Apple is one of Wall Street's most innovation-driven tech stocks, Buffett's attraction to Berkshire's No. 1 holding by market value has everything to do with its premier capital-return program and customers' loyalty toward Apple's physical products, including iPhone, Mac, and iPad. With this being said, it's unlikely to be a tech stock.

Removing tech and healthcare leaves 54 potential stocks Buffett could be purchasing in secret in either the energy or industrial sectors.

Though purchasing an energy stock in secret is more plausible than a tech stock in Buffett's portfolio, it's still unlikely. Energy has rarely played a large role in Berkshire's investment portfolio dating back to the start of the century. At the moment, combined investments in Chevron and Occidental Petroleum have given Buffett's company some of its largest energy exposure of the century. With two integrated oil and gas companies already among Berkshire's largest holdings, the addition of a third large energy stock seems improbable.

The signs would appear to point to an industrial sector company, which would speak to Buffett's roots of investing in American manufacturing companies.

Last month, I laid out my case why United Parcel Service (NYSE: UPS) is most likely the secret stock Warren Buffett is buying. UPS is a former Buffett stock that maintains clear-cut competitive advantages, has a robust capital-return program, and is historically cheap.

However, there are other potentially inexpensive industrial stocks that I can't eliminate as possible purchase candidates. For instance, UPS rival FedEx (NYSE: FDX) is another logical buy candidate. Logistics companies represent the heart of the American economy, and Buffett has strongly cautioned investors not to bet against America. In recent quarters, FedEx has found itself on better financial footing than UPS.

Though Buffett hasn't made a sizable investment in a defense company in a long time, Lockheed Martin (NYSE: LMT) might make sense. Lockheed losing out on a contract to build the next-generation of fighter jets for the Air Force to Boeing has created a temporary price dislocation that might speak to Berkshire's billionaire chief.

It could even be something as simple as heavy construction giant and Dow Jones Industrial Average component Caterpillar (NYSE: CAT) . While Buffett isn't a fan of mining companies, he's also not oblivious to companies like Caterpillar that offer large-scale competitive advantages.

Buffett's mystery stock is a big (brand-name) company that isn't listed in Berkshire's 13F, and it's more than likely a company you'll find in the industrial sector. But narrowing it down any more than this is a guessing game.

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Citigroup is an advertising partner of Motley Fool Money. Bank of America is an advertising partner of Motley Fool Money. Sean Williams has positions in Bank of America. The Motley Fool has positions in and recommends Apple, Bank of America, Berkshire Hathaway, Chevron, and FedEx. The Motley Fool recommends Constellation Brands, Lockheed Martin, Occidental Petroleum, and United Parcel Service. The Motley Fool has a disclosure policy .

Billionaire Warren Buffett Is Buying Shares of a Mystery Stock: Here's Everything We Know was originally published by The Motley Fool