Several lysosomal acid hydrolases were assayed in peripheral blood leukocytes from a patient with I-cell disease by the method of Hindman, J. and Cotlier, E. ((1972) Clin. Chem. 18, 971--975). The activities of lysosomal hydrolases in polymorphonuclear cells showed no significant differences between the patient, the parents, and normal controls, while lymphocytes from the patient exhibited the reduced activities of alpha- and beta-galactosidases, beta-glucuronidase, and N-acetyl-beta-glucosaminidase. After phytohemagglutinin-stimulated culture, lymphocytes from the patient showed a more definite reduction in the activities of these lysosomal enzymes; the activities of beta-glucuronidase, alpha-mannosidase, and N-acetyl-beta-glucosaminidase were reduced to about 20--30% of the activity in the phytohemagglutinin-stimulated control cultures, and the activities of alpha- and beta-galactosidases to 10% or less. Lymphocytes from the parents showed no significant difference in the activities of these enzymes from the controls, whether stimulated or not.