Vitamin D3, 1 alpha,25(OH)2D3, and its metabolites regulate the growth and differentiation of several cell types. Vitamin D3 and its analogue, calcipotriol (MC 903), inhibit the proliferation of cultured human and mouse keratinocytes and induce keratinocyte differentiation. Calcipotriol is effective in the treatment of psoriasis in which increased plasminogen activator activity has been reported. We analyzed therefore the effects of calcipotriol and vitamin D3 on the production of plasminogen activator (PA) activity in human keratinocytes and a mouse keratinocyte cell line. Caseinolysis-in-agarose assays indicated that vitamin D3 decreases total PA activity in both keratinocyte culture systems. Zymographic analyses of the medium indicated that the secreted activator was of the urokinase type (u-PA). A decrease was observed also in extracellular matrix and membrane-associated u-PA activity of vitamin D3 and calcipotriol treated cells. Immunoblotting analysis of the conditioned medium from human keratinocytes revealed a decrease in the u-PA protein levels. Accordingly, Northern hybridization analysis of the respective mRNAs indicated a rapid decrease in urokinase mRNA levels. Calcipotriol decreased u-PA activity also in the presence of inducers of u-PA activity like transforming growth factor-beta, epidermal growth factor, and phorbol-12-myristate-13-acetate. Calcipotriol also caused a decrease in tissue type PA (t-PA) activity of the keratinocytes. Most t-PA activity was associated with the extracellular matrices and cell membranes as revealed by zymographic analysis. Paradoxically, the secretion and deposition of the matrix of plasminogen activator inhibitor type 1 decreased in calcipotriol-treated cells. The results indicate that a major effect of vitamin D3 on cultured keratinocytes is a decrease of plasminogen activator activity.