Incoming type C retroviral genomic 35S RNA is present in polysomes of undifferentiated and differentiated murine teratocarcinoma cell lines at 4 hours after infection. At the same time a 65,000 daltons viral specific protein is produced by the infected cells. These data present evidence that incoming viral RNA serves as messenger for the synthesis of gag protein precursor Pr65 early in the infectious cycle of ecotropic murine retrovirus.