The effect of the tumor promoter phorbol myristate acetate (PMA) on phagosome-lysosome (P-L) fusion in mouse macrophages has been studied using a previously described (10) fluorescence assay. Treatment with 0.1--1.0 microgram PMA/ml caused a striking increase in the rate and extent of P-L fusion. Exposure of cells to phorbol, free myristate, or the monoesters of PMA did not reproduce this effect. Macrophages required from 2 to 3 h of pretreatment to express maximal P-L fusion, and this was maintained for at least 20 h when cells were returned to PMA-free medium. Catalase, superoxide dismutase, indomethacin, and hydrocortisone, agents that are known to block the effect of PMA on H2O2, O2-, prostaglandins, or plasminogen activator, did not affect the stimulation of P-L fusion by PMA. The protein-synthesis inhibitors puromycin and cycloheximide did block the PMA effect under conditions in which the high fusion rate of 4-d cells was not affected. Labeled PMA was rapidly taken up by macrophages, with a plateau of uptake at approximately 3 h. When cells were returned to PMA-free medium, cel-associated label was rapidly released, returning to background level within 1 h. The released label was found to be a metabolite of PMA by thin-layer chromatography. This product migrated between the monoester phorbol-12-myristate and free phorbol. Rapid metabolism of PMA was also observed by a macrophage cell line, J774, and, to a lesser extent, by primary rat embryo fibroblasts.