We studied the effect of c-jun depletion in Friend murine erythroleukemia (F-MEL) cells stably transfected with a plasmid that allowed for the glucocorticoid-mediated conditional expression of c-jun antisense sequences. The c-jun cDNA used for the construction of the vector was modified so as to prevent the nonspecific targeting of junB and junD transcripts. High level and rapid induction of c-jun antisense transcripts was achieved with as little as 10(-8) mol/L dexamethasone (DEX) and resulted in a 80% to 90% reduction in c-jun protein levels. The continuous exposure of the transfected cells to DEX inhibited growth by greater than 85% over a 5-day period, whereas DEX had no effect on the growth rate of control F-MEL cells. This proliferative block was associated with a reversible accumulation of cells with a 2n DNA content. When these cells were recultured in the absence of DEX, c-jun protein rapidly reappeared and the immediate early response genes egr-1, junB, and c-myc were transiently expressed. Thus, inhibition of c-jun protein causes logarithmically growing cells to leave the cell cycle and to enter a state closely resembling, if not identical to, G0. These results underscore the importance of c-jun in maintaining cellular proliferation and provide additional evidence for the participation of proto-oncogenes in cell cycle control.