Summary. The occurrence of cytotoxic lymphocyte subpopulations (i.e., CD 16 ÷, CD57 ÷ and cytotoxic CD 8 ÷) was studied in the peripheral blood of 18 B-cell chronic lymphocytic leukemia (B-CLL) patients. The absolute numbers of CD 57 ÷, CD 16 ÷ and cytotoxic CD 8 ÷ lymphocytes were increased in the peripheral blood of untreated patients as compared with healthy donors, suggesting a causal relation with the accumulation of malignant B-cells. For 5 B-CLL patients and 5 hematological normal donors, the lymphocyte subpopulations in peripheral blood, lymph nodes and bone marrow were determined. A significant immune response was observed in the lymph nodes of the patients, as reflected by the CD 3 ÷ lymphocytes, which were 1.7 -2 7 times larger in the patients lymph nodes than in their peripheral blood and bone marrow. In contrast, with peripheral blood this was mainly caused by an increase in CD 4 + lymphocytes. The CD 57 lymphocytes in the lymph nodes of the patients had abnormal orthogonal light-scattering signals and an abnormal density of CD 57 ÷ receptors in comparison with their peripheral blood CD 57 ÷ lymphocytes or the CD57 ÷ lymphocytes in the peripheral blood, bone marrow and tonsils of the hematological normal donors. This study shows that although a significant increase of cytotoxic lymphocytes in the peripheral blood of B-CLL patients is observed, the actual distributions of the non-malignant lymphocytes can be quite different at the actual t u m o r sites, i.e., bone marrow and lymph nodes.