The stabilities and translation of Ehrlich ascites tumor cell poly(A)-containing mRNA and mengovirus RNA in fractionated cell-free protein synthesizing systems from uninfected and mengovirus-infected Ehrlich ascites tumor cells were studied. During incubation of the systems about 20% of the input RNA is reduced in size and associated with ribosomes engaged in polypeptide synthesis; the remainder is rapidly degraded by RNases. At the end of active translation, both mRNA and nascent proteins are bound to polysomes which are of the same size as those formed during active protein synthesis. The kinetics of protein synthesis closely follow those of RNA hydrolysis. The stabilities of mengovirus RNA and poly(A)-containing mRNA from Ehrlich ascites tumor cells are the same in both systems.