A description is given of a rat anti-mouse hybridoma antibody, JIId, which reacts with erythrocytes, neutrophils, greater than 90% of thymus cells, and most B cells. JIId does not have detectable activity for mature T cells, pluripotential stem cells, platelets, or cells of the monocyte-macrophage lineage. Although the JIId antigen is present on 90 to 95% of typical small B lymphocytes, pretreatment of spleen cells with JIId plus complement has no effect on secondary IgG antibody responses; by contrast, primary IgM responses and proliferative responses to lipopolysaccharide are substantially reduced. Unlike the precursors of IgG antibody-producing cells (AFC), IgG AFC per se are strongly JIId-positive. Rapid acquisition of the JIId antigen also applied to the early progeny of pluripotential stem cells.