We have performed a comparative study of the regulation by glucocorticoids and phorbol 12-myristate 13-acetate (PMA) of the production of type 1 plasminogen activator inhibitor (PAI-1) by 12 human cell lines. A sandwich-type enzyme-linked immunosorbent assay (ELISA) for PAI-1 that measures free PAI-1 as well as complexes between PAI-1 and both types of plasminogen activators has been used. Basal PAI-1 accumulation varied more than 5000-fold between the cell lines. No correlation was found between the PAI-1 level and other characteristics of the cell lines, except that three lines of SV40-transformed fibroblasts produced more PAI-1 than two non-transformed fibroblast cell lines. Three out of the 12 cell lines responded to glucocorticoids by an increased PAI-1 production. Four cell lines responded to PMA by an increased PAI-1 production. In addition, PMA-induced differentiation of the monocyte cell line U937 and the promyelocytic cell line HL-60 into macrophage-like cells was found to be correlated with an up to 100-fold increase in PAI-1 accumulation. The PMA-dependent differentiation of HL-60 cells led to acquisition of glucocorticoid inducibility of PAI-1. These findings provide information for future studies of the molecular mechanism of cell-specific expression and regulation of PAI-1.