Monoclonal antibodies (mabs) to SCUPA have been generated in balb/C mice by conventional means and have been demonstrated to have no crossreactivity with two chain urinary-type plasminogen activator (TCUPA). These mabs have been used to develop two types of specific assay for SCUPA. Mabs coated on polyvinyl plates in conjunction with polyclonal antibodies (pabs) to TCUPA have allowed the development of a catcher-tag ELISA using alkaline phosphatase-label led goat anti-rabbit IgG as a final step. The sensitivity range of the assay was 0.5 - 10.0 iu/ml. A second bioimmunoassay (BIA) using SCUPA mabs as catcher and inplate development with glu-plasminogen and S-2251 has yielded an assay with a sensitivity range of 0.5 - 10.0 iu/ml. The international unitage ascribed in these assays was derived by comparing the hydrolysis of S-2444 by the I.S. for TCUPA with the purified SCUPA following full activation with plasmin.Using these assays it was found that normal pooled plasmas contained about 1.0 iu of SCUPA antigen which was fully inhibited such that no activity was evident by the BIA assay for SCUPA. This suggests that urokinase in plasma is present in two forms: SCTJPA bound to inhibitor and TCUPA which is biologically active when assayed using a BIA based on immobilised pabs to TCUPA. Cell supernatants from cultured human lung fibroblasts yield a SCTJPA/TCUPA ratio of 70/30 using S-2444 chromogenic assay following a plasmin-mediated SCTJPA-TCUPA conversion step. It was also shown that, in these cell supernatants, SCTJPA was secreted with no inhibitor bound to it, since the BIA and ELISA data were quite similar. A curious feature of these assays, which is as yet unexplained, is the observation that urokinase (both plasmin activated SCTJPA and TCUPA), when immunologically adsorbed on to the PVC-immobilised specific mabs or pabs used in this study, readily activated plasminogen but showed no hydrolytic activity on the chromogenic substrate, S-2444.