ABSTRACTTwenty‐three pediatric patients with severe aplastic anemia were evaluated using the assay for colony forming units in culture. In co‐culture experiments, bone marrow from 5 of the 23 patients suppressed normal bone marrow growth greater than 50%. Of the 11 patients who subsequently received immunosuppressive therapy, consisting of antithymocyte globulin, five patients obtained a clinical response. The presence or absence of suppression in the co‐culture assay was not predictive of the clinical outcome. Although autoimmune mechanisms may indeed be responsible for the aplasia in certain cases of severe aplastic anemia, furtherin vitrostudies need to be developed to identify patients in whom autoimmune mechanisms are operative.