A novel subset of human blood lymphocytes was isolated by means of labelling with monoclonal antibodies and fluorescence-activated cell sorting. In normal individuals, the new subset accounts for about 2% of the blood T lymphocytes. The cells of this subset bind monoclonal antibodies specific for T lymphocytes in general [e.g. OKT3, Hu-Lyt 3(9 . 6) and Leu-22] and they also form E rosettes. However, no binding is seen with monoclonal antibodies to T-lymphocyte subsets (OKT4, OKT8, Leu-2A and Leu-3A). Moreover, the lymphocytes of this new subset express neither Ia antigens nor membrane immunoglobulins. They do not bind OKM1, an antibody against cells of the myelomonocytic lineage that also reacts with natural killer cells, nor do they bind OKT6 or OKT10, specific for thymocyte antigens. The cells have a high specific gravity, a thymocyte-like pattern of lactate dehydrogenase isoenzymes and do not contain terminal deoxynucleotidyl transferase. Although these lymphocytes are viable, also after culture in vitro, and can be stored in liquid nitrogen, they are inert in all functional systems tested: they neither proliferate upon stimulation with mitogens or allogeneic cells, nor do they display suppressor or natural killer cell activity. A patient who was successfully reconstituted by bone marrow transplantation for severe combined immunodeficiency, was found to contain an abnormally high (25%--30%) fraction of these OKT3 positive, OKT4 and OKT8 negative cells among his circulating T lymphocytes.