Reproduction of mouse encephalomyocarditis virus (EMC) was studied in 5 continuous primate cell lines: HeLa, Fl, Detroit-6, P/7, and MIO inoculated with guanidine-dependent variant of poliomyelitis virus in the absence of guanidine. Poliomyelitis virus stimulated EMC virus reproduction in all cell lines under study. This stimulation effect was studied at length in HeLa and MIO cells. In HeLa cells, stimulation was observed at a low and moderate multiplicity of infection of EMC virus but not at a high (100 PEU/cell) multiplicity. Also, when EMC virus reproduction was stimulated, a shortening of the latent period of its multiplication cycle, an increase in the number of antigen-containing cells and the number of infectious centers were observed. In MIO cells, stimulation was found to occur both with low and high doses of EMC virus but not to be accompanied by a shortening in the latent period of EMC reproduction cycle, or any increase in the antigen-containing cells or number of infectious centers. In both cell types upon mixed infection the synthesis of virus-specific RNA's of EMC virus was enhanced. It is suggested that the stimulating effect of poliomyelitis virus is realized in HeLa and MIO cells at different stages of EMC virus reproduction.