Summary The properties of enzyme systems of E. coli and of rat liver, which degrade β-mercaptopyruvate, have been compared. The bacterial system was much more stable to heat and less susceptible to oxidation. These characteristics have been related to lack of dependence upon sulfhydryl group. At the pH optimum for pyruvate formation, the enzyme activity may be characterized as a transsulfurase rather than a desulfurase. When cyanide is the sulfur acceptor, thiocyanate formation occurs. The pH optimum for E. coli system is 9.6 and for rat liver system 9.1.