Ron Radosh says he was:

In my very first week of blogging, I wrote about the revered late left-wing journalist, I.F. Stone. Sure, Izzy charmed a lot of his supporters. But as I noted, he was most well known for being an apologist for Stalinism, and a journalist who at the time of the Korean War, perpetrated Soviet disinformation that the war was started by South Korea with the backing of the United States.

Until now, there has been only highly circumstantial evidence indicating that for several years Stone may have been a KGB agent. Now, John Earl Haynes, Harvey Klehr, and Alexander Vassiliev in their soon to be published book, Spies:The Rise and Fall of the KGB in America, present new evidence that indeed this was the case. Actual KGB files they examined, scrupulously copied from the originals by Vassiliev, offer us proof that from 1936 to 1938, Stone was in fact a Soviet agent. The chapter giving the data now appears on the website of Commentary magazine.| MORE ›