Peripheral blood lymphocyte subsets were studied in 6 Chediak-Higashi patients and 12 family members. The lymphocyte subsets were characterized by monoclonal antibody reagents and fluorescence flow-cytometry. An increase in OKT8 (suppressor/cytotoxic) and a decrease in OKT4 (helper) cell populations was observed in all patients studied. No correlation was seen between the clinical status (presence or absence of the lymphoproliferative phase) and the percentage of the lymphocyte subsets. The patient's mothers also had an increased percentage of OKT8-positive cells. The significance of these findings is discussed regarding the patients clinical course.