Murine hybrid hybridomas secreting bispecific monoclonal antibodies (bs mAbs) were constructed by fusing hybridomas secreting an anti-tissue plasminogen activator (tPA) or anti-urokinase (UK) mAb with hybridomas secreting a mAb which binds to human fibrin but not to fibrinogen. The bs mAbs reactive to both fibrin and PA (tPA or UK) were purified by affinity chromatography employing the respective antigen-coupled columns and characterized by fibrin-binding, amidolytic and fibrinolytic assays. Immunochemical conjugation of PAs and the bs mAbs did not impair the catalytic activity of PAs at all and made it possible to concentrate PAs on fibrin clot. Pretreatment of fibrin with the bs mAbs enhanced the fibrin-binding of PAs and the subsequent fibrinolysis.