Primary cultures of mouse epidermal cells (i.e., target cells of skin tumor promotion) stimulated by 12-O-tetradecanoylphorbol-13-acetate (TPA) release prostaglandin E2 within 30 min. Anthralin, a non-TPA type tumor promoter, also stimulated PGE2 release; however, no release was detectable at least up to 4 hr after the addition of anthralin. When the cells were incubated with TPA plus anthralin, both PGE2 and arachidonic acid release were synergistically enhanced. Other non-TPA type tumor promoters, i.e., chrysarobin, 7-bromomethylbenz[a]anthracene, benzoylperoxide, okadaic acid and palytoxin, did not potentiate the TPA-caused PGE2 release. In protein kinase C-down regulated cells, the synergistic stimulation of PGE2 and arachidonic acid release by TPA plus anthralin were not detected. Anthralin plus TPA did not alter the incorporation of arachidonic acid into cellular phospholipids. Cellular cyclooxygenase activity was increased 2 hr after TPA stimulation. Anthralin-caused increase in cyclooxygenase activity was detected at 6 hr after the addition of anthralin. Cyclooxygenase activity was synergistically increased by treating the cells with TPA plus anthralin. Cycloheximide and actinomycin D inhibited the increase in cyclooxygenase activity caused by anthralin or TPA plus anthralin. These results indicate that anthralin synergistically stimulates TPA-caused PGE2 release by synergistically increasing arachidonic acid release and cellular cyclooxygenase activity.