Recently, we showed that localization of Glu-plasminogen on cell surfaces enhances its conversion to Lys-plasminogen by exogenous plasmin. This leads to stimulation of plasminogen activation because Lys-plasminogen is the preferred substrate on cell surfaces. Here, we show that Glu-plasminogen was converted to Lys-plasminogen on monocytoid cells in the absence of exogenous plasmin. Culture of cells under serum-free conditions did not affect this conversion, suggesting that the enzymatic activity was cell-derived. Therefore, we tested whether endogenous monocytoid plasminogen could provide a source of plasmin to convert cell-associated Glu-plasminogen to Lys-plasminogen because plasmin is the only enzyme known to effect this reaction. We used a recombinant human plasminogen mutant, [D(646)E]Pg, which can be cleaved by plasminogen activators, but cannot catalyze the generation of Lys-plasminogen. Upon incubation with either THP-1 or U937 monocytoid cells, 35 and 38%, respectively, of the cell-bound ligand was converted to Lys-[D(646)E]Pg. Trasylol, alpha2-antiplasmin, and an anticatalytic antiplasminogen monoclonal antibody decreased Lys-[D(646)E]Pg formation to < 5% on monocytoid cells, consistent with a plasmin-dependent mechanism. Plasminogen was detected in these cells by Northern blotting and RT-PCR. Our results suggest that plasmin converts cell-bound Glu-plasminogen to Lys-plasminogen and that this enzyme is produced by activation of monocytoid plasminogen by endogenous monocytoid plasminogen activators to enhance plasminogen activation on the monocytoid cell surface.