Papain (EC 22.214.171.124) and succinyl-papain exhibit isoelectric points of 9.6 and 4.3, respectively. The acid limb of the pH activity curve in the pH range of 3.5–4.5 of native papain and the basic limb in the pH range of 8–9 of succinyl-papain are shifted by monovalent salts, whereas the basic limb of native papain and the acid limb of succinyl-papain are not shifted by KCl. The Km and not the kcat with benzoylglycine ethyl ester as substrate is affected by salt. The order of the effectiveness of the ions is I- > NO3- > Br- > Cl- and Li+ > K+. These effects can be explained in terms of non-specific binding of cations and anions and electrostatic interactions.