Rat 3Y1 cells were infected by AKR virus through microinjection of molecularly cloned proviral DNA. Based on a strong immunofluorescence using anti-p30 as an antiserum a cell clone (RESA-2) was selected that had a high expression of viral antigens. Subsequent restriction analysis of its DNA revealed that the RESA-2 clone contained at least 30 apparently intact integrated proviruses per genome. There was an apparently normal synthesis and processing of gag and pol gene products. The viral envelope precursor polyprotein gPr82env, however, did not yield the major envelope glycoprotein gp70. The gag precursor polyprotein, Pr65gag, as well as the gPr82env from RESA-2 cells were identified as AKR viral proteins by gel electrophoresis of hydroxylamine cleavage fragments. The virions formed by RESA-2 cells lacked gp70 and were noninfectious. After fusion of RESA-2 cells and mouse cells an infectious N-tropic virus was produced. The results indicate that rat 3Y1 cells lack (a) factor(s) necessary for the correct processing of gPr82env. The high incidence of abortive infections of murine leukemia virus (MLV) in susceptible rat cells reported by others is therefore probably due to defective particles in the virus stock and/or to the lack of (a) cellular factor(s) necessary for reverse transcription and subsequent integration of the viral genome.