The hydrolysis and peptide formation reactions of carboxypeptidase Y (CPDY) were studied in a ternary system consisting of octyl beta-D-glucopyranoside (beta-OG), water, and octanol (OcOH). This was performed to elucidate the effects of the structures of the surrounding medium on biocatalytic reactions, with special reference to enzyme reactivity and the morphology of the surfactant aggregates. First, the effect of beta-OG on the catalytic activity of CPDY (hydrolytic reaction) was studied in a surfactant/water binary system. beta-OG was found to inhibit CPDY activity for Suc-Ala-Ala-Pro-Phe-pNA, but somehow accelerated the reaction with Bz-Tyr-pNA. When the CPDY reactions were measured under several conditions in the phase diagram of the ternary system, CPDY was found to be strongly affected by the size and form of the microstructures in each phase. In the reverse micelle (L(2)) phase, the hydrolytic rate was maximally increased with an increase in the water content; the highest rate was observed at the boundary of the L(2)/2L or L(2)/F (reversed cylindrical diameter) phase. In the aminolysis reaction catalyzed by this enzyme, a higher yield of the dipeptide was also observed near the boundary of the L(2)/2L or L(2)/D + X phase and also in the F phase. The fraction of aminolysis (f(a)) was relatively high and rather insensitive to changes in the phase, with the exception that the f(a) was low in the D/X phase.