Icahn wrote, "To Bill Gross @PIMCO: If you really want to do good, why not join givingpledge.org like Gates, I and many others have?"
The Giving Pledge is a commitment by several of the world's wealthiest people, including Bill Gates and Warren Buffett, to dedicate the majority of their money to philanthropy.
(Read more: Pimco's Gross tells Icahn to leave Apple alone )
Icahn's latest barb comes days after Pimco's Gross Tweeted on Thursday, "Gross: Icahn should leave #Apple alone & spend more time like Bill Gates. If #Icahn's so smart, use it to help people not yourself."
Last Thursday in a public letter to Apple CEO Tim Cook, Icahn called on the tech giant to commence a $150 billion share buyback immediately.
-By CNBC's Katie Little. Follow her on Twitter @KatieLittle
image: © Daniel Dudek-Corrigan