SummaryNine lysosomal enzyme activities were examined in granulocytes and lymphocytes from two unrelated patients with Chediak-Higashi syndrome (CHS) in “accelerated phase” and from their family members. In CHS granulocytes, there was a marked reduction of α-mannosidase (E.C. 3.2.1.24), α-galactosidase (E.C. 3.2.1.22), and α-fucosidase (E.C. 3.2.1.51) activities, which were below 21, 24, and 43% of mean control values, respectively. In CHS lymphocytes, β-glucuronidase (E.C. 3.2.1.31) and α-mannosidase activities were also decreased.In granulocytes of family members, the activities of acid phosphatase (E.C. 3.1.3.2),N-acetyl-β-glucosaminidase (E.C. 3.2.1.30), aryl sulphatase (E.C. 3.1.6.1), and β-glucuronidase were significantly higher than the control values (P< 0.001), which were 262, 218, 414, and 180% of mean control values.Neither the inhibitor in CHS granulocytes nor the activator in the heterozygous granulocytes to those enzymes could be found by mixing experiments with normal ones.