Since polyclonal B-cell activators (PBA) require macrophages to induce modifications in the lymphocyte in vitro primary response to thymus-dependent antigens, we have investigated whether PBA act directly on macrophages. [14C]glucosamine uptake by guinea pig peritoneal adherent cells after stimulation with purified protein derivative (PPD), lipopolysaccharide (LPS), and dextran sulfate (DxS) was tested. PPD produced an increased [14C]glucosamine uptake, whereas LPS and DxS did not. According to our experiments, (a) PPD does not require the presence of lymphocytes to stimulate macrophages; furthermore, when lymphocytes were present in a concentration higher than 5%, a suppressor effect in the glucosamine uptake was found, and (b) there was no significant difference between the findings when peritoneal adherent cells were cultured in normal medium and in supernatant of lymphocyte cultures stimulated with PPD.