The reactions of alpha 2-macroglobulin (alpha 2M) with plasmin or streptokinase-plasmin(ogen) (SkPl) were studied as a function of temperature. alpha 2M and plasmin reacted relatively rapidly at all temperatures. The initial rates of reaction were identical at 24 and 37 degrees C. At 4 degrees C, the initial reaction rate was somewhat slower; however, the reaction was 90% complete in 2 min. The reaction of alpha 2M with SkPl was markedly temperature-dependent. Initial rates of reaction at 0 and 24 degrees C were only 3 and 40% of the rate of 37 degrees C, respectively. When these reactions occur, only the plasmin moiety is incorporated in the alpha 2M molecule. These data explain the inconsistencies in previous reports of SkPl interactions with alpha 2M, since experiments have been performed at different temperatures in various studies. In the present work, we present a model which interprets the data in terms of a possible high-energy transition state in the alpha 2M-SkPl reaction.