Purification of O-acetylserine sulfhydrylase (OASS) from seedlings of two species of Phaseolus reveals the presence in both species of two forms of this enzyme. The isolation and purification procedure gives purification of 7- to 160-fold for individual isoenzymes with specific activities ranging from 33 IU mg(-1) to 775 IU mg(-1) protein.Detailed study of the basic kinetic parameters of the OASS isoenzymes indicates that both forms from Phaseolus vulgaris (which are of about equal specific activity) display substrate inhibition by S(2-) above 1 mm and positive cooperativity at lower concentrations of S(2-). With respect to O-acetylserine (OAS), the second substrate of the reaction, one P. vulgaris isoenzyme shows substrate inhibition by OAS concentrations above 10 mm, while the second is unaffected by OAS concentrations up to 50 mm. The isoenzymes from Phaseolus polyanthus (one of which has a specific activity 24 times higher than the other) are slightly and approximately equally inhibited by both S(2-) and OAS.