Characterization of the different lymphocyte populations in normal human bone marrow (BM) was attempted and compared to that in the peripheral blood (PB). B [bone marrow-derived] cells comprised 34% .+-. 11% of lymphocytes in BM and 23% .+-. 9% in PB. The majority of B cells carried Ig [immunoglobulin] M in BM and IgG in the PB. In the BM, cells carrying complement or Fc receptors were fewer than cells carrying Ig, but in the PB they were equal. T [thymus-derived] cells comprised 6% .+-. 4% of lymphocytes in the BM and 62% .+-. 7% in the PB. The majority of BM lymphocytes did not have B or T cell markers; these probably included B and T cell precursors. BM lymphocytes carrying surface Ig increased in a 7-day culture; those of the PB decreased. Pokeweed mitogen induced Ig synthesis in B cells of PB but not those of BM. BM-T cells were more efficient than PB-T cells in inhibiting Ig synthesis of PB-B cells. The BM compartment probably contains immature B cells that are capable of partial differentiation and maturation in vitro. BM-B lymphocytes are probably not involved in the effector phase of the immune response since they are unable to synthesize Ig and because they carry few receptors for complement or Fc. BM-T lymphocytes are very few and have suppressor capability and may play an essential role in regulation of Ig synthesis by B cells.