Investigation of in vitro humoral immune responses and immunoregulatory properties of leukemic cell was carried out in 17 children with acute leukemia prior to therapy. Leukemias were of the non-T, non-B-cell type in 13 patients and of T-cell origin in four. Bone marrow and peripheral blood cells consisted of 24-96% lymphoblasts and were generally deficient in surface Ig-positive cells. Induction of Ig secreting cells in response to pokeweed mitogen was markedly decreased in marrow and peripheral mononuclear cell cultures of leukemic patients. Co-culture of leukemic cells with normal lymphocytes led to marked deviations from the expected Ig secreting-cell response of the cell mixtures. The predominant effect was enhancement, as was the case with eight non-T, non-B-cell and one T-cell leukemia samples. Suppression of the Ig secreting-cell response was observed in only three instances, two with non-T, non-B-cell and one with T-cell leukemia samples. These findings implicate non-T, non-B as well as more differentiated leukemic cells in having the potential for modifying Ig production by B cells.