Abstract The enhancing effects in vitro of a potent skin-tumor promoter, phorbol myristate acetate (PMA), and its derivatives on nitroblue tetrazolium (NBT) reduction, phagocytosis and cell spreading of mouse peritoneal macrophages were compared. The enhancing effects on these markers of macrophage function, especially NBT reduction, were found to correlate well with the skin-tumor promoting activities of the phorbol esters. Another potent promoter, teleocidin, strongly enhanced NBT reduction, although teleocidin is structurally different from phorbol esters. Other tumor promoters, namely saccharin, phenobarbital, cantharidin and Tween 60 did not enhance NBT reduction by macrophages. These data indicate that this test system is appropriate for screening of PMA-type tumor promoters such as phorbol esters and teleocidin.