Incubation of radiolabeled immature Moloney murine leukemia virus (Mo-MLV) in buffers containing the reducing agent dithiothreitol (DTT) or adsorption to NIH 3T3 cells resulted in similar time-dependent cleavage of the Pr65 Gag poly-protein by the endogenous viral protease (PR). The combined results suggest that cell membrane-associated thioreductases may initiate and accelerate retroviral polyprotein processing and that redox regulation may be involved in virus maturation, which is essential for replication.