Summary Polyacrylamide-gel electrophoresis of foot-and-mouth disease virus disrupted with 8m-urea revealed the presence of five major protein bands and one minor one. Evidence was obtained suggesting the likelihood of these bands being due to the presence of distinct polypeptides in the virus protein. Virus which had lost 3 to 4 log. of infectivity by treatment with trypsin also yielded six bands in polyacrylamide-gel electrophoresis. Five of these bands had the same mobility and intensity as those found in untreated virus but the slowest-moving band in the untreated virus had an enhanced mobility after enzyme treatment, suggesting that only one of the constituent polypeptides of the virus protein was affected by the enzyme. This polypeptide is necessary for the attachment of the virus to susceptible cells and for the immunizing activity of the inactivated virus particle.