Berkshire previously said it has a much smaller stake in Activision, which is bought by one of the portfolio managers who help Buffett invest Berkshire’s money.
Speaking at Berkshire’s annual meeting, Buffett said he was responsible for the additional investment, and that Berkshire would file a report with regulators if its stake in Activision exceeded 10%.
Berkshire revealed earlier on Saturday that it spent more than $51 billion on stocks in the first quarter.
It has made multibillion-dollar investments this year in oil companies Chevron Corp and Occidental Petroleum Corp, as well as computer and printer maker HP Inc.
Berkshire also agreed in March to pay $11.6 billion in cash for insurance company Alleghany Corp.