In humans with acute thrombotic disease, thrombi often appear to resist fibrinolysis induced by plasminogen activators. To examine the potential role of alpha 2-antiplasmin (alpha 2AP) in thrombus resistance in vivo, we generated monoclonal antibody inhibitors of alpha 2AP. In a somatic cell fusion, 99 hybridomas were obtained that produced MAbs that bound to human 125I-alpha 2AP in a capture assay. Screening assays showed that 3 of these MAbs, 49, 70, and 77, neutralized the function of alpha 2AP. Immunoblotting experiments indicated that these MAbs recognized an epitope present in native alpha 2AP that was destroyed by denaturation with SDS. Each of these MAbs fully inhibited the binding of the other MAbs to alpha 2AP, but none of them competed with the binding of another anti-alpha 2AP MAb, RWR. When tested for their binding to nonhuman alpha 2APs in plasmas, all three MAbs were strongly crossreactive with all primate plasmas tested but showed an idiosyncratic pattern of binding to alpha 2AP in other plasmas, suggesting unique fine epitope specificities. In human plasma, all three MAbs amplified the lysis of human plasma clots induced by plasminogen activators, increasing the potency of urokinase by nearly 50- to 100-fold. These MAbs also markedly amplified the lysis of clots from baboon, cynomolgus, african green monkey plasmas, and to a lesser extent, ferret and dog. By virtue of their ability to potently inhibit alpha 2AP in other animal plasmas, these MAbs should be useful for examining the role of alpha 2AP in thrombus resistance to fibrinolysis in vivo.