The spleen from a patient with adult Gaucher's disease was shown to be deficient in a β-glucosidase (EC 3.2.1.21) isoenzyme that has optimal activity at pH 4.0-4.3, and is stimulated by 0.02% Triton X-100. A mixture of spleen homogenates from a control and from the patient contained β-glucosidase activity equivalent to 2-3 times the theoretical expected activity. The increase in enzyme activity occurred at pH 4.0-4.3; the magnitude of the increase was proportional to the amount of each homogenate added. Two factors, one called factor P from the patient's spleen, the other called factor C from the control spleen, were responsible for a reconstitution of β-glucosidase activity in vitro. Factor P is tentatively identified as an acid glycoprotein.