A study was made of the virus-specific proteins present in Krebs 2 ascites cells infected with encephalomyocarditis (EMC) virus. About 15 virus-specific polypeptides, mol. wt. varying from 13000 to 147000, were detected by acrylamide gel analysis of extracts from infected cells briefly pulsed with [14C]-amino acids. Three of the components were probably capsid proteins, while the fourth capsid protein (7300 mol. wt.) was not detected in the infected cell and possibly arose after maturation by cleavage of the largest capsid protein. Pulse-chase experiments showed that only two medium-sized (mol. wt. 36000 and 54000) and some smaller (mol. wt. 13500 to 17000) non-capsid proteins were stable in vivo; the remaining larger non-capsid proteins lost radioactivity during the chase with cold amino acids, and were probably intermediates in the formation of smaller polypeptides by some post-translational cleavage process. Various methods were employed to block this cleavage process, utilizing elevated temperatures, protease inhibitors and amino acid analogues.