19 patients with chronic 'warm type' autoimmune hemolytic anemia were studied for abnormalities of cellular immune reactions. Evidence was obtained for a reduction of rosette-forming cells (RFC). Lymphocytotoxic antibodies were present in only 8 patients and correlated, with only one exception, with a reduced number of RFC. No significant deviation from normal ranges of the three major immunoglobulin classes in the patients' sera were found. C3 and C4 complement components were also, with one exception, within normal limits. In 18 of 19 patients no apparent association existed between the type or the amount of autoantibodies and/or complement components fixed on red cells and the levels of the respective immunoglobulins or complement in the sera.