Concentrations of IgA, IgG, and IgM were measured in 25 patients with Gaucher's disease. The mean of each was significantly higher than that generally observed in healthy adults. A direct correlation was noted between IgA and IgG concentrations and age. Patients who underwent splenectomy had IgM concentrations significantly lower than those of persons who did not. In two of 25 patients, multiple myeloma of the nonsecretory type developed. The migration inhibition factor (MIF) test performed with the lymphocytes of our patients was positive to glucocerebroside in four of 17 patients, to glucocerebrosidase in four of 19 patients, and to the extracts of the spleen of a patient with Gaucher's disease in three of eight patients. The results of al MIF tests were negative in six control subjects with splenomegaly of other causes. Our results suggest that in Gaucher's disease there is chronic stimulation of the humoral immune system. The first expression of this stimulation is the production of polyclonal immunoglobulins and, in due time, the development of monoclonal immunoglobulin as well as multiple myeloma.