In May 1957, he was called to testify at Arthur Miller's trial for contempt of Congress. He was Miller's "expert witness on communism" and he testified that he "did not believe" that Miller had written his plays "under the discipline of the Communist Party". His testimony was unusual in that normally only the government produced 'expert testimony' to demonstrate that the defendant was a Communist. In January 1964 he testified in a libel trial brought on behalf of John Goldmark, a Washington state legislator who had been defeated partially on the basis of allegations that his membership in the ACLU was tantamount to being a member of a Communist-front organization. Cain testified that the ACLU had never been on the Attorney General's list of such organizations and Goldmark won a sizable award from the defendants.