It has previously been shown that the RNA replicase of encephalomyocarditis virus contains two virus-coded proteins, D and E, which are produced in two successive proteolytic steps: (i) C leads to D + ?; and (ii) D leads to p22 + E. It is here shown (i) that virus protein H (molecular weight, 12,000) is the previously unidentified product of the first step and (ii) that VPg, a protein linked covalently to the virion RNA, yields two tryptic peptides found in protein C but not in protein D. The results suggest that VPg is derived by cleavage of protein C and that protein H may be intermediate. Preliminary experiments with VPg sequences in polioviral noncapsid protein 1b, the counterpart of encephalomyocarditis viral protein C, were inconclusive.