Human null cells were isolated from peripheral blood lymphocytes (PBL) by differential centrifugation on a Ficoll-Hypaque gradient of the PBL following simultaneous rosetting with erythrocyte indicators specific for B and T lymphocytes. Specifically, the T lymphocytes were rosetted with 2-aminoethylisothiuronium bromide-treated sheep erythrocytes (ShE), whereas the B lymphocytes were either rosetted with ShE coated with xenoantibodies against human gamma globulin or first sensitized with monoclonal antibodies to human Ig-like antigens and then rosetted with ShE coated with xenoantibodies against mouse gamma globulin. Approximately 90% of the lymphocytes isolated were null cells that did not bear detectable B-cell markers-that is, surface immunoglobulin and/or Ia-like antigens-or T-cell markers-that is, ShE receptors. The large majority of the null cells expressed receptors for the IgG Fc fragment (53-93%), C3 component (65-92%) and monkey erythrocytes (60-91%) but lacked receptors for the IgM Fc fragment and murine erythrocytes. The null cells exhibited high natural killer cell activity and antibody-dependent cellular cytotoxicity and were two- to four-fold active than the total PBL and the T-enriched cell fractions. The null cells, however, did not respond to stimulation with phytohaemagglutinin and failed to function as either stimulators or responders in an unidirectional mixed lymphocyte reaction.