A distinctive subpopulation of nonphagocytic, tightly adherent cells (NPAC) comprised approximately 6% of the adherent peritoneal cells from untreated mice, and about 18% of those from mice previously given BCG i.p. A separation procedure based on adherence and lack of phagocytosis was devised. Isolated NPAC were morphologically intermediate between small lymphocytes and macrophages. They were positive for nonspecific esterase, negative for peroxidase, positive for surface IgM, and negative for surface IgG1, IgG2 and IgA. When capped, their surface IgM regenerated in vitro. NPAC had demonstrable Fc receptors but not EAC receptors. They resisted killing by an anti-macrophage serum, were negative by immunofluorescence with an anti-T cell reagent, and incorporated increased amounts of thymidine in response to LPS but not to PHA. They were more readily killed with anti-Ia serum and complement than macrophages, but less readily than splenic B cells. NPAC appeared to represent a subpopulation of B lymphocytes which contaminates some preparations previously regarded as "macrophages" and which may be ressponsible for some of the activities previously ascribed to "macrophages".