An appreciable percent (3-14%) of the lymphocyte-like cells of the mouse spleen lack both the theta-isoantigen and sufficient surface immunoglobulin to be detected by conventional immunofluorescence or autoradiographic procedures. These theta(-),Ig(-) cells are increased in frequency after treatment of mice with antithymocyte serum or in mice that have been thymectomized, irradiated (850 R), and reconstituted with bone marrow cells. Moreover, in chimeric C57BL/6 mice in which the T cells are derived from (BALB/c x C57BL/6)F(1) donors, theta(-),Ig(-) cells also lack BALB/c histocompatibility antigens. These experiments indicate that theta(-),Ig(-) cells are not theta(-) T lymphocytes. Removal of complement receptor lymphocytes from spleen cell populations increases the frequency of theta(-),Ig(-) cells, indicating that such cells lack the complement receptor. Partially purified populations of theta(-),Ig(-) cells have been obtained by cytolysis by anti-theta- and anti-kappa-antibody and complement and by density gradient ultracentrifugation. These cells closely resemble lymphocytes in morphology. The only exceptional feature is the existence of prominent nucleoli. The theta(-),Ig(-) cells lack hemoglobin and endogenous peroxidases, are not actively phagocytic, and do not adhere to glass. This suggests they are not of the erythroid, myeloid, or monocytoid lines. [(3)H]Thymidine labeling studies indicate that theta(-),Ig(-) cells are members of a relatively slowly dividing cell pool. Whether theta(-),Ig(-) cells are members of the "classical" B lymphocyte line or belong to another, as yet undescribed, lineage is not yet certain.