Two isozymes of membrane-bound β-glucosidase (β-d-glucoside glucohydrolase, EC 126.96.36.199) with activity towards 4-methylumbelliferyl-β-d-glucopyranoside have been identified in human cells. One of these isozymes was found to have a pH optimum of 5.0, a Km of 0.4 mM and to be rapidly inactivated at pH 4.0 ("acid-labile"). The second isozyme had a pH optimum of 4.5, a Km of 0.8 mM and was stable at pH 4.0 ("acid-stable"). Cultured long-term lymphoid lines and peripheral blood leukocytes contained both isozymes while cultured skin fibroblasts contained only the "acid-stable" form in detectable amounts. The specific activity of the "acid-stable" isozyme was severely reduced in cultured skin fibroblasts, cultured long-term lines and peripheral leukocytes from patients with Gaucher's disease. The specific activity of the "acid-labile" enzyme in the latter two cell types was apparently unaffected. The β-glucosidase activity in all three cell types examined was predominantly particulate but the enzyme could be solubilized with low concentrations of Triton X-100. The solubilized enzyme required sodium taurocholate (0.2%) for maximum activity. Solubilized β-glucosidase did not exhibit the cell-specific differences in pH optimum and Km shown by the membrane-bound enzyme.