Twenty-five patients with aplastic anemia, 8 female and 17 male aged from 16 to 79 year-old, were examined their circulating lymphocyte subpopulations and the responses to mitogens in vitro.T-lymphocyte was identified using sheep erythrocyte rosette-formation test under the Nikon phase contrast microscope. Surface immunoglobulin bearing cell was identified under the Nikon fluoromicroscope as B-lymphocyte using membrane immuno-fluorescence test. As mitogens in vitro PHA-P (Difco Lab.) and pokeweed mitogen (Grand Island Biol. Co.) were added to the lymphocyte culture during 68 hours in the CO(2) incubator following 4 hours flushing with (3)H-TdR. The incorporated cpm was counted with millipore filtration and a liquid scintillation counter (Shimazu). According the severity of aplastic anemia, the patients showed decreased circulating lymphocytes, decreased T- and B-lymphocytes with increased null cells. The incorporation of (3)H-TdR in vitro in the soloculture decreased remarkably as well as PHA-P responsiveness. Increased bone marrow T-lymphosytes were noticed in those patients. Although the higher cpm of soloculture of the bone marrow lymphocyte was obtained than of the circulating lymphocyte, extremely low responses to PHA-P and pokeweed mitogen were characteristic in the aplastic bone marrow lymphocytes. The splenectomy corrected T- and B-lymphocytes population in contrast to the persistent low responsiveness to mitogens. The abnormality of lymphocyte subpopulations and incorporation rate of (3)H-thymidin stimulated with pokeweed mitogen and PHA-P might effect on bone marrow cells as cytotoxic or cytolytic.