The in vivo synthesis of encephalomyocarditis-specific proteins was studied by labeling the viral proteins with radioactive amino acids under conditions where host-protein synthesis was almost completely inhibited. To assure recovery of all proteins, intact cells were lysed in hot 1% sodium dodecyl sulfate. These lysates were analyzed by quantitative high-resolution electrophoresis on sodium dodecyl sulfate-polyacrylamide gels. This technique allowed the detection and estimation of the molecular weight of 15 virus-specific polypeptides: A, 100,000; B, 90,000; C, 84,000; D, 75,000, D1, 65,000; E, 56,000; epsilon, 40,000; F, 38,000; alpha, 34,000; beta, 30,000; gamma, 23,000; G, 16,000; H, 12,000; I, 11,000; and delta, 9,000. Pulse-chase experiments, in conjunction with cyanogen bromide and tryptic mapping of the isolated polypeptides, indicate that at least three primary gene products (A,F,C), with a cumulative weight of about 220,000, are generated during translation of the RNA genome. Chains A and C then undergo post-translational cleavages, while F remains uncleaved. The proteins generated by the cleavage of A include all of the capsid chains (alpha, beta, gamma, delta, epsilon). Those generated by the cleavage of C include D and E. The chains alpha, beta, gamma, delta, E, F, G, H, I, with a cumulative molecular weight of about 230,000, are stable and are produced in about equimolar amounts. A model for the synthesis of, and a cleavage sequence that accounts for, all of the viral polypeptides is proposed.