Strains of foot-and-mouth disease virus of types O1 and A10 were isolated which showed no significant loss of infectivity upon trypsinization. These 'trypsin-resistant' (TR) viruses were obtained by serial passage in BHK cells of virus that was trypsin-treated before inoculation of the cells. Three O1 isolates were cloned and studied further. Cell attachment of those TR O1 variants (OTR1) was not reduced by trypsinization, unlike that of parent virus. The polypeptide compositions of TR viruses as determined by SDS-polyacrylamide gel electrophoresis were identical with those of parent virus, with the exception of OTR1 which contained an additional polypeptide approx, 3000 daltons larger than VP1. After trypsinization, which normally cleaves VP1, the polypeptide composition of the three TR viruses (including OTR1) and of parent virus did not show any significant difference. In OTR1 both the additional virus protein and VP1 were cleaved into a P18 molecule and smaller fragments. The surface location of this additional polypeptide was confirmed by iodination experiments. It was shown by immunodiffusion experiments that only OTR1 differed from the parent virus. This antigenic change was present on the trypsin-sensitive part of the virus since trypsinized TR viruses (including OTR1) were antigenically identical to trypsinized parent virus. The electrophoretic mobilities of the three OTR viruses isolated, and of parent virus, differed somewhat before trypsinization. After trypsin-treatment, the mobilities of TR viruses were all increased to the same level; however, their rate of migration was lower than that of trypsin-treated parent virus. This lower mobility of trypsin-treated OTR viruses was the only difference which could be associated with retained infectivity.