Antibody-dependent cellular cytotoxicity (ADCC) by cells from peripheral blood was studied in patients with: chronic lymphocytic leukaemia (CLL), chronic granulocytic leukaemia (CGL), acute lymphoblastic leukaemia (ALL), acute non-lymphoid leukaemia (ANLL) and compared with that of healthy donors. The ADCC activity in all the types of leukaemia was on the average lower as compared with the control values, being especially low in both CLL and CGL. Further, it has also been shown that the ADCC activity of non-leukaemic blood cells in ANLL was lower as compared with control values. The latter observation indicates that, at least in ANLL, the reduced ADCC potential of peripheral blood cells is not due to dilution of effector cells by poorly-or non-active leukaemic cells.