The interactions of porcine alpha2-macroglobulin (alpha2M) with native proteinases, their zymogens and the chemically-modified enzymes were compared. The alpha2M did not bind to chymotrypsinogen, or to most of the chemically modified derivatives of alpha-chymotrypsin, trypsinogen, DIP- and PMS-trypsins, but it could interact with anhydrotrypsin, PMS-subtilisin, and O-acetylated neutral subtilopeptidase. Anhydrotrypsin appeared to bind very tightly to alpha2M, as does native trypsin, whereas the binding of PMS-subtilisin to alpha2M was weaker than that of the native enzyme, judging from exchange experiments with labeled enzyme and from competitive enzyme assay. There are, however, some differences in the mode of interaction with alpha2M between native and anhydrotrypsins. (1) The shape and the magnitude of ultraviolet difference spectra caused by the interaction with alpha2M were significantly different. (2) The interaction of alpha2M with active proteinase led to the formation of new amino-terminal amino acids, while that with anhydrotrypsin did not. (3) In vivo experiments showed that radioactivity of 3H-labeled trypsin-alpha2M complex was rapidly cleared from the plasma of rats, whereas the anhydrotrypsin-alpah2M complex was cleared very slowly. These results suggest that the proteolytic activity of the enzyme is not obligatory for the first phase of alpha2M-proteinase interaction (formation of Michaelis-type complex), but only the proteolytically modified complex is cleared rapidly from the blood circulation system.