It has previously been shown that when encephalomyocarditis viral RNA is translated in cell-free extracts of rabbit reticulocytes, it synthesizes a virus-coded protease, p22, which is derived by cleavage of a precursor protein, C. Protein C is shown here to be cleaved by two different mechanisms, which were distinguished by their sensitivity to dilution. One mechanism was sensitive to dilution; the other was not. The biphasic cleavage behavior was unchanged by diluting incubation mixtures with untranslated reticulocyte extract instead of buffer, suggesting that both types of cleavage were mediated by virus translation products. It is proposed that the dilution-sensitive cleavage of protein C is due to a virus-coded protease, probably p22 itself, and that the dilution-independent cleavage is due to intramolecular self-cleavage of protein C.