1. 1. Papain is reversibly inactivated in the presence of air and low concentrations of cysteine. This inactivation is enhanced by Fe2+ and Cu2- and is retarded by EDTA. In the absence of cysteine, active papain (separated from the activators by gel filtration) is inactivated at a much lower rate in an almost irreversible manner. 2. 2. Papain inactivated in the presence of cysteine has exactly the same ate of activation as papain which is inactive after its preparation according to Kimmel and Smith. It is concluded that these papains are identical. 3. 3. Experiments with radioactively labelled cysteine have shown that reversibly inactivated papain is a mixed disulphide of papain and cysteine. This cysteine is removed on activation. 4. 4. Some experiments with divalent metal ions have shown that Cd2+ has a strong, Zn2- a very strong preference for inhibitory combination with papain over binding with cysteine. Fe2+, Cu2+, Hg2+ and Pb2+, show no or a small preference for papain over cystein. On the other hand, in the absence of cysteine, Zn2+ is removed from papain by EDTA, but not Hg2+.