Streptokinase-plasmin complex (SkPl) was prepared with human plasminogen. Regulation of SkPl and plasmin by the plasma proteinase inhibitors, alpha 2-antiplasmin (alpha 2AP) and alpha 2-macroglobulin (alpha 2M), was studied as a function of temperature in plasminogen-depleted human plasma, mouse plasma, and solutions of purified proteins. The reaction of plasmin with proteinase inhibitors in human plasma was complete. alpha 2AP was the predominant inhibitor. The fraction of alpha 2M-plasmin recovered was not affected significantly by incubation temperature. In contrast, the reaction of SkPl with human proteinase inhibitors was markedly temperature dependent. The apparent second-order rate constant for the reaction of SkPl with purified alpha 2AP at 37 degrees C (1.5 x 10(2) mol/L-1 s-1) was greater than 150-fold higher than the constant derived at 4 degrees C. In human plasma and in solutions containing mixtures of purified human proteins, alpha 2AP was the principal inhibitor of SkPl. Elevating the temperature enhanced the reaction of SkPl with alpha 2AP and alpha 2M comparably. Equivalent results were obtained when incubations were performed in platelet-rich plasma (PRP) or whole blood. In murine plasma, SkPl reacted readily with the proteinase inhibitors. The principal inhibitor of SkPl was alpha 2M. Maximum reaction between SkPl and murine alpha 2M was observed at 37 degrees C; however, significant reaction also occurred at 4 degrees C. alpha 2 AP was the predominant inhibitor of plasmin in mouse plasma. Reaction of alpha 2AP with SkPl in murine plasma was significant only after the alpha 2M was inactivated with methylamine. These results were not affected by platelets or whole blood cells. We conclude that the thrombolytic efficacy of streptokinase reflects not only the nature of the plasminogen activator complex but also the function of the proteinase inhibitors.