Studies have been carried out on the activities and properties of the isozymes of α-mannosidase, α-glucosidase and β-glucosidase in granulocytes, monocytes, lymphocytes and platelets from peripheral blood of healthy adult donors. The findings reveal the differences in activities as well as a characteristic distribution of the different molecular forms of these lysosomal hydrolases in specific cell types. Therefore, the results obtained with unfractionated total leukocyte samples from different subjects may vary according to the distribution of cell types in the circulation. Granulocytes and monocytes show only the acid α-mannosidase activity whereas lymphocytes and platelets show both acid and neutral activities. The specific activity of acid α-mannosidase in granulocytes and monocytes is higher than in lymphocytes and platelets. By DEAE-cellulose chromatography, the acid α-mannosidase in granulocyte and monocyte extracts elutes as two peaks, but only one peak is seen in lymphocytes. All cell types show both acid and neutral α-glucosidase activities. The specific activities of both isozymes are higher in granulocytes and monocytes than in lymphocytes and platelets. Monocytes show a higher acid than neutral activity. All other cell types show a higher neutral activity. β-Glucosidase in all cell types is mainly membranebound and it can be released by Triton X-100 and sodium taurocholate. Taurocholate also stimulates the β-glucosidase activity of granulocytes, monocytes and lymphocytes whereas it inhibits the activity of this enzyme in platelets. These results indicate that variations in the total number of leukocytes and in the relative proportion of the various cell types in health and disease may yield inconsistent or unreliable values for enzyme activity in the diagnosis of ly sosomal storage disease and in carrier detection