Peripheral blood lymphocytes from 14 patients with idiopathic aplastic anemia were examined to determine their capacity to differentiate into immunoglobulin (Ig)-producing cells in the presence of pokeweed mitogen (PWM). Four patients had hypogammaglobulinemia. Lymphocytes from 10 patients without and 1 with hypogammaglobulinemia were capable of differentiating into Ig-producing cells. Lymphocytes from 3 patients with hypogammaglobulinemia were unable to differentiate. Coculture experiments showed that in 1 patient helper activity of T [thymus-derived] cells was impaired. In another patient B [bone marrow-derived] cells could not be stimulated by PWM to differentiate even in the presence of normal T cells. In a 3rd patient the helper function of T cells and the capacity of B cells to differentiate into Ig-producing cells were affected. Differentiation of lymphoid cells is probably impaired in some cases of idiopathic aplastic anemia associated with hypogammaglobulinemia.