In this study the effects of vitamin D metabolites on the myelomonocytic precursor cell line U937 have been compared with those of phorbol myristate acetate (PMA). PMA was used as a cell modulating agent in order to avoid effects of binding of the exogenous vitamin D metabolites receptors within the cell, which would interfere with subsequent measurement of these receptors in studies of the vitamin D3 metabolic pathway. Both the active 1,25 DHCC form of vitamin D3 and the inactive 24,25 DHCC metabolite inhibit cell proliferation and induce 24-hydroxylase activity, but not 1 alpha-hydroxylase activity. These effects are dose-dependent and maximum enzyme activity is seen in the adherent cell population, which is induced by these compounds. PMA inhibits proliferation of U937 and increases receptors for 1,25 DHCC in these cells (like the vitamin D metabolites). However, unlike the vitamin D metabolites, PMA induces 1 alpha-hydroxylase activity rather than 24-hydroxylase activity. Thus, while PMA and 1,25 DHCC have some similar effects on monocyte precursor cell line differentiation, there is a difference between the effects of the two agents on the vitamin D metabolic pathway. The former promotes synthesis of the active metabolite, and the latter induces an enzyme which renders the metabolite inactive. If these results are considered together, they are consistent with the hypothesis that 1,25 DHCC has an autocrine role within the mononuclear phagocyte system.