The effect of a variety of inhibitors of prostaglandin synthase, lipoxygenase and phospholipase on the growth of P815 murine mastocytoma cells was examined. Only lipoxygenase inhibitors substantially reduced growth, presumably by inhibiting the production of arachidonic acid metabolites rather than causing arachidonate accumulation since excess arachidonic acid did not reverse growth inhibition. Evidence is presented that production of leukotrienes B4, C4, D4 or E4 was not involved. Other metabolites of arachidonic acid were not excluded. A role for lipoxygenase in growth signal transduction in these and other cells is suggested.