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Warren Buffett just lost about $1 billion on this

Warren Buffett does not like to lose money in general, so losing $1 billion before lunch on a Monday morning can not be going down well.

The plunge in IBM shares Monday after its weak earnings results cost the Oracle of Omaha dearly. The stock fell $13.06, and Buffett held about 70.2 million shares as of June 30, according to the most recent SEC filings.

That means the sharp decline cost him $916.5 million—a drop in the Berkshire Hathaway bucket, to be sure, but still noteworthy. (At IBM's lows of the day, he was down as much as $1.08 billion).

Read MoreIBM on earnings: We're disappointed

In April, after a prior weak earnings report, Buffett told CNBC he had not "soured" on IBM, that he had bought more stock this year and that he had not sold a share.

The stock was recently the third-largest holding in Buffett's portfolio, trailing only Wells Fargo and Coca-Cola.

IBM CEO Ginni Rometty dismissed talk of the tech company splitting up, despite its large size, Monday's earnings miss and the recent fad for tech companies to split in two.

Read MoreRometty: Clear path forward

"There is no doubt that marketplace speed has increased," she said in a CNBC interview. "We have a very clear strategy about how to take this company to the future."