Berkshire Hathaway Inc., Buffett's investment company, bought 9.81 million Apple shares last quarter. Apple's stock has been trending downward since last summer.
As of March 31, the date of a Berkshire regulatory filing describing the purchase, the shares were worth $1.07 billion.
Investors have questioned Apple's ability to launch new hit products and generate revenue from services in recent months. Apple shares are currently trading in the low $90-range, after starting the year at $104.15.