Inhibitory activity directed against metalloenzymes has been highly purified from extracts of red kidney beans (Phaseolus vulgaris). The inhibitor is a substance of small molecular weight and appears to be a chelator of Zn(2+). One milligram of the preparation inhibited 23 milligrams carboxypeptidase A. The inhibitor also strongly inhibited carboxypeptidase B and alkaline phosphatase and could activate phosphoglucomutase that had previously been inactivated with Zn(2+). The isoelectric point of the inhibitor is 4.7. The inhibitor activity was abolished by preincubation with Zn(2+), Ni(2+), Co(2+), or Cu(2+). The mechanism of inhibition of carboxypeptidases and alkaline phosphatase by the bean inhibitor is apparently due to the complexing and complete removal of Zn(2+) from the enzymes.