The constituents of LS antigen from cells infected with vaccinia virus and with cowpox virus were compared by immunoprecipitation and SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Antiserum to the LS antigen from cells infected with vaccinia virus reacted with at least five polypeptides in cells infected with either virus. Four of these polypeptides were similar sizes in cells infected with the two viruses. However, one major polypeptide with a mol. wt. of about 100 000 (100 K) detected in cells infected with vaccinia virus was not found in cells infected with cowpox virus. Conversely, a polypeptide with a mol. wt. of about 60 000 (60K) was detected only in cells infected with cowpox virus.