Phorbolol myristate acetate, a metabolite of the tumor promotor phorbol muristate acetate in mouse skin, has one-fiftieth the potency of the parent molecule for the induction of cell division in stationary cultures of BALB/c-3T3 mouse embryo cells. Similarly, in a mixed cell culture assay devised for detection of tumor-promoting agents, phorbolol myristate acetate exhibited only a small fraction of the activity of unmetabolized phorbol ester. The results indicate that the biological activity of phorbol esters either does not require metabolic conversion or is converted by the cells used in this system and that phorbolol myristate acetate would be a tumor promotor of low potency for mouse skin compared to phorbol myristate acetate.