Sephadex G-100 gel filtration of a purified mixture of tryptophan-synthase-inactivating enzymes I and II from Saccharomyces cerevisiae, which had previously been identified as yeast proteinases A and B, yields two coincident peaks of tryptophan-synthase-inactivating activity and proteinase A and B activities. The peaks correspond to molecular weights of approximately 100 000 and 50 000. It could be demonstrated, with purified proteinases A and B, that the high molecular weight peaks is due to the formation of a stoichiometric complex between proteinases A and B. The AB complex can easily be separated into its constituent enzymes by DEAE-Sephadex chromatography. When in the AB complex, proteinase B molecules still bind the specific proteinase B inhibitor from yeast.