The effects of various concentrations of a water-miscible organic solvent [a 7:3 (v/v) mixture of N, N dimethylformamide and dimethylsulfoxide] on the kinetics of papain have been investigated. The parameters kcat and Km for the amidase and esterase activity of papain using N-α-benzoyl-DL-arginine-p-nitroanilide (BAPNA) and N-α-benzoyl-L-arginine ethyl ester (BAEE) as substrates were determined. For both types of activity, kcat initially increased (up to about 15% solvent), and then decreased with increasing concentrations of organic solvent. In contrast, Km increased sharply with the organic solvent concentration. Active site titration at 0 and 50% solvent indicated no change in the amount of active enzyme. Fluorometric measurements of the emission spectrum of papain did not indicate any major conformational changes with increasing concentrations of organic solvent.