Tumor-promoting agents are known to inhibit the specific differentiation processes of several animal cell systems in vitro, including the Friend leukemia cell system. We have examined the effect of 12-O tetradecanoylphorbol-13-acetate (TPA) on the latter system and have investigated its action on Friend virus expression. At a concentration of 16.7 nM, TPA inhibits the dimethyl sulfoxide-induced Friend cell terminal differentiation and, at the same time, enhances the expression of the Friend virus genome, as demonstrated by a 2-fold increase in the amount of reverse transcriptase-containing particles released into the culture fluid and in the levels of virus-specific intracytoplasmic RNA. The greatest effect of TPA is evident after 24 hr of treatment. At this time, TPA exerts also its strongest effect upon the induction of the plasminogen activator. Our results indicate that two specific effects of TPA, i.e., block of differentiation and induction of plasminogen activator, correlate well in the Friend cell system with an extracellular and intracellular increase in virus expression.