Immunoprecipitates of BHK21 cells infected with aphthovirus C and harvested at 60, 65, 75, 90, 120 and 210 min post-infection (p.i.) and the corresponding rabbit hyperimmune serum were analyzed by polyacrylamide gel electrophoresis. Inhibition of cellular protein synthesis was not observed. The immunoprecipitates contained polypeptides with a MW lower than 22,500, different from those obtained from BHK-anti BHK rabbit serum. In all the antigenic samples the antibodies raised in rabbits detected the polypeptides of the virus which was replicating in infected cells. As time p.i. elapsed the polypeptides made evident an increase in radioactivity up to 75 min, a decrease between 75 and 120 min and a new increase at 210 min. This outcome is in agreement with a former hypothesis which pointed out that up to 75 min p.i. there would be non released and penetrating virus, between 75 and 120 min p.i., a probable prelytic liberation of early viral progeny and from 120 min p.i. liberation of viral progeny and alteration of the fibroblast. Control immunoprecipitates of BHK cells and rabbit antiserum to different samples in increasing time p.i. and of those samples with anti BHK rabbit serum were analyzed. These controls confirmed the working hypothesis, and the existence in BHK cells of low MW polypeptides in low concentration, which increase in the samples due to viral action. Rabbit antiserum to infected cells harvested at 75 and 210 min p.i. mainly precipitated polypeptides of MW 120,000-72,000, 63,000-56,000 and 52,000-34,000. This result is coincident with the fact that structural polypeptides precursor is responsible to induce protective antibodies in neonatal mice.