Although the infection of BHK cells with foot-and-mouth disease virus did not cause a marked inhibition of cellular protein synthesis, the proteins synthesized gradually changed from host-specific to virus-specific. The synthesis of at least thirteen virus-induced proteins was demonstrated by polyacrylamide electrophoretic analysis of the infected cells. Only a small proportion of the virus-induced proteins was incorporated into the mature virus particles. The addition of amino acid analogues caused an accumulation of the larger mol. wt. proteins, suggesting that in infected cells, some cleavages occurred, giving rise to the smaller mol. wt. proteins. The operation of the precursor-cleavage mechanism was more clearly shown in pulse-chase experiments, in which it was found that the proteins were degraded from larger to smaller mol. wt.