Differential plasminogen activation on U937 and THP-1 cells. Plasmin generation is shown for U937 cells (A-B) and THP-1 cells (C-D) after incubation of the cells with either pro-uPA (A,C) or active, 2-chain uPA (B,D). The data are shown fitted to integrated rate equations describing the plasminogen activation system and using previously published kinetic constants. In the case of U937 cells, the data are consistent with 100% of the bound uPA being either in the active or pro form; in the case of THP-1 cells, the best fit of the data in the presence of pro-uPA is achieved by assuming that 50% of the bound uPA is initially present in the activated form. Plasminogen activation was not detected on cells preincubated with the blocking anti-uPAR monoclonal antibody R3.