This study was undertaken to determine the proportions and in vitro immune capacities of lymphocyte populations in the bone marrows of normal humans. Relatively pure mononuclear cell suspensions were obtained from bone marrow aspirates by linear sucrose gradient centrifugations. Simultaneous peripheral blood and bone marrow specimens from each individual were assayed for lymphocyte surface markers and mitogen responsiveness. Maximal possible contamination of bone marrow aspirates by peripheral blood was determined by performing aspirates on individuals who had received 51chromium-labeled autologous erythrocytes. Rhymus-derived (T) lymphocytes, as determined by the sheep red blood cell (E) rosette assay, comprised 8.6-(plus or minus 1.6)% of the total bone marrow lymphocyte pool. Bone marrow-derived (B) lymphocytes, as determined by the presence of a complement receptor, made up 15.4-(plus or minus 1.9)% of the lymphocyte pool whereas 74.6 (plus or minus 2.4)% of mononuclear cells lacked easily detectable surface markers. These findings could not be explained by contamination with peripheral blood lymphocytes since contamination was corrected for in the calculations. Lymphocyte-enriched suspensions of bone marrow cells responded to stimulation with phytohemagglutinin, concanalin A, and particularly pokeweed mitogen. In vitro incubations of bone marrow and peripheral blood lymphocytes with tritiated thymidine followed by determinations of E and erythrocyte antibody complement (EAC) rosettes were performed. Simultaneous rosetteradioautographs demonstrated that the proliferative potential of bone marrow B lymphocytes was greater than peripheral blood B lymphocytes (P less than 0.01). On the other hand, the proliferative potential of bone marrow T lymphocytes was the same as that of peripheral blood T lymphocytes. These findings demonstrate that in addition to containing B lymphocytes the normal bone marrow contains a small fraction of T lymphocytes similar to the mature T lymphocyte pool found in the peripheral blood. These T cells most probably enter the bone marrow parenchyma as part of the normal recirculating lymphocyte pool.