While investigating an involvement of other factors aside from endogenous IL-3 and prostaglandin E (PGE) in mast cell induction from mouse splenocytes, we found that the mast cell induction was inversely proportional to IL-4 levels and tended to directly proportionate IFN-gamma levels in the supernatants recovered on days 2 and 4. Thereafter, we examined the effects of rIFN-gamma, rIL-4, and rIL-10 on mast cell induction. IFN-gamma and IL-10 dose-dependently induced mast cells. Time course study showed an importance of adding rIFN-gamma into the cultures at the early phase (on days 0 and 2 of a 12-day culture). When endogenous IFN-gamma at the early phase was neutralized by anti-IFN-gamma Ab, all stimulants, including rIFN-gamma, rIL-10, and PGE1, failed to induce mast cells. On the contrary, rIL-4 dose-dependently suppressed the mast cell induction by rIFN-gamma, rIL-10, LPS, PGE, and dibutyryl cAMP. The inhibitory effect of IL-4 was observed when IL-4 was added into the cultures at the early phase, but not after day 4. The suppressive action of IL-4 was diminished completely by the addition of neutralizing anti-IL-4 Ab. IL-12, a key regulator of IFN-gamma and IL-4 production, also induced mast cells. These results revealed, for the first time, that IFN-gamma is crucial for the survival and/or differentiation of splenic mast cell precursors and that IL-4 is a key inhibitor for the precursors, although IFN-gamma is not a mast cell growth factor and IL-4 is a growth factor for immature and mature mast cells.