To generate bispecific monoclonal antibodies, reactive to both fibrin and tissue-type plasminogen activator (t-PA), we planned to generate anti-t-PA monoclonal antibodies (mAb) which eliminate negative aspects of t-PA such as the inhibition by plasminogen activator inhibitor-type 1 (PAI-1) and the rapid clearance of t-PA. Here we report on the isolation and characterisation of a set of 13 mAb against t-PA, some of which meet the above requirements. Apart from their potential in the production of bispecific antibodies, these and the other mAb can be useful in structure-function analysis and a variety of other applications. Experiments involving PAI-1 showed that one mAB (12-5-3) reacts only with free t-PA, and prevents the subsequent binding of PAI-1 to mAb-bound t-PA. In vitro studies on the receptor mediated uptake of t-PA by hepatic cells, showed that one mAb (1-3-1) specifically inhibited the association of t-PA with liver endothelial cells. Other tests showed that mAb 7-8-4 and 12-5-3, but not 1-3-1, inhibited in vitro the enzymatic activity of t-PA. On the basis of these and other observations, we conclude that especially mAb 1-3-1, and in vivo possibly 7-8-4 and 12-5-3 may be good candidates for incorporation in bispecific monoclonal antibodies.