The interaction of alpha2-macroglobulin (alpha2M) with an alkaline serine proteinase (ALPase 1) from alkalophilic Bacillus sp. grown in an extraordinarily alkaline environment was investigated. Stoichiometry of the reaction showed that ALPase I bound to alpha2M in a molar ratio of about 2:1. The alpha2M-ALPase I complex showed about 80% of the proteinase activity shown by ALPase I in the hydrolysis of succinyl-L-alanyl-L-alanyl-L-prolyl-L-phenylalanyl-4-methyl-coumaryl-7-a mide (Suc-Ala-Ala-Pro-Phe-MCA) and casein. The conformational changes in the alpha2M molecule caused by the complex formation at pH 7.5 were determined from electron micrographs and difference spectra. The antigenic activity of the alpha2M-ALPase I complex with the anti-ALPase I antiserum was found to be completely abolished. Immunoelectrophoresis of the complex incubated at pH 7.5 after 48 h showed no appreciable change, and the complex was recognized as exhibiting enhanced stability at pH 7.5.