Extracts of wheat germ are capable of synthesizing the major capsid protein of simian virus 40. Poly (A) containing RNA from BS C1 cells infected with simian virus 40 directed the synthesis of a novel polypeptide that migrates in polyacrylamide gels together with the major capsid polypeptide of simian virus 40, VP 1. The patterns of the major tryptic peptides of purified VP 1 and the novel polypeptide synthesized in vitro were identical after two dimensional paper electrophoresis. The novel polypeptide was not synthesized in response to poly (A) rich RNA from uninfected cells or from virus infected cells treated with cytosine arabinoside. Messenger RNA from infected cell purified by selective hybridizattion to DNA of simian virus 40 directs the synthesis of a major polypeptide of electrophoretic mobility similar to that of VP 1 of simian virus 40. This approach should prove useful in identifying additional products specified by DNA tumor viruses.