Thirteen children with systemic lupus erythematosus (SLE) and 5 with juvenile dermatomyositis (JDM) were studied for natural cytotoxicity (NK) and antibody-dependent cellular cytotoxicity (ADCC). In vitro prednisolone at pharmacological levels failed to alter NK or ADCC in adult controls, nor did it alter ADCC in any children studied. ADCC was normal, but NK was decreased in SLE (p less than .005) and JDM (p less than .05) compared to controls. Since NK but not ADCC is decreased in these patients, different lymphocyte populations may mediate these 2 cytotoxic functions.