Mononuclear phagocytes are known to produce both urokinase-type plasminogen activator (u-PA) and a specific PA inhibitor, PAI-2. In this study we have investigated the effects of calcitriol and PMA-induced differentiation on the comparative expression of PA and PAI in monoblast-like U937 cells. Cells were incubated for 96 h in the presence or absence of 50 nM calcitriol. After transfer into serum-free medium, the cells were cultured for 48 h with PMA (0 to 50 ng/ml). PA and PAI activities of conditioned media and cell lysates were measured with a plasminogen dependent colorimetric assay. Control cells uniformly secreted PAI activity (70.3 +/- 24.9 PAI U/ml), while calcitriol pretreatment induced the cells to secrete PA activity (52.6 +/- 47.2 milli-Ploug unit/ml). PMA induced secretion of PAI activity in calcitriol-pretreated cells to levels 4.6- to 8.3-fold greater than controls (p less than 0.05). Parallel effects on PA and PAI activities were seen in cell lysates. To determine how changes in the expression of u-PA and PAI-2 might account for these effects, mRNA for u-PA and PAI-2 were assessed by Northern blot analysis. Calcitriol induced an increase in u-PA mRNA with a marked reduction in PAI-2 mRNA. PMA alone induced modest increases in both mRNA species. In calcitriol pre-treated cells, PMA induced a moderate increase in u-PA mRNA and a marked increase in PAI-2 mRNA. We conclude that agonist-specific differentiation of U937 cells modulates the expression of PA and PAI activities by altering the proportionate biosynthesis of u-PA and PAI-2 proteins. The ability of mononuclear phagocytes to control plasminogen activation at inflammatory foci may therefore be contingent on the independent regulation of u-PA and PAI-2 gene expression.