The early cell surface antigen (ECSA) induced by vaccinia or cowpox virus infection was analysed by immunoprecipitation and SDS-polyacrylamide gel electrophoresis (SDS-PAGE), The components with a mol. wt. of about 40 000 (40 K) in vaccinia virus-infected cells and with a mol. wt. of about 43 000 (43 K) in cowpox virus-infected cells were specifically immunoprecipitated with anti-ECSA serum prepared by immunizing rabbits with rabbit kidney cells infected with a conditional lethal mutant. The 40 K and 43 K polypeptides were synthesized even in the presence of cytosine-I-beta-D-arabinofuranosyl-HCl (Ara C) and were detectable even in the cells pulse-labelled between I and 2 h p.i.