The rate of conversion of dissolved substrate by a suspension of enzyme crystals is governed by the rate of diffusion and the reaction rate of the substrate inside the crystal. If the crystal is thin enough the diffusion is not rate limiting. This critical size, dc, equals vKmDi/kcat[E] where Di is the diffusion rate of the substrate, and [E] the enzyme concentration inside the crystal. The value of dc increases if either a substrate with a high value for Km/kcat (i.e., a poor subtrate) or crystals having low [E] values (mixed crystals of mainly irreversibly inhibited enzyme and a small quantity of active enzyme, etc) are used. These principles were applied to two crystal modifications of papain, A and C. As a poor substrate acetylglycine ethyl ester was used; benzoylarginine ethyl ester was used in conjunction with mixed crystals of 98% papain inactivated with chloroacetamide and 2% active papain. With both substrates and both crystal modifications complete activity was observed when the effect of the suspension medium (20% Na2SO4) upon the reaction rate was taken into account. This latter effect was evaluated with cross-linked papain crystals insoluble in both water and 20% Na2SO4. Data from the literature concerning three crystalline hydrolytic enzymes are discussed in terms of the above and other equations involving Di, as derived in the appendix. It is concluded that chymotrypsin and ribonuclease-S crystals may, like papain, be completely active. A convenient method of preparing mercuri-papain and S-carboxyamidomethylene papin is described.