Thoracic duct lymph (TDL) of nonimmune rats and mice was examined for the presence of antigen-carrying cells immediately following a single injection of 125I-labelled or fluorescence-labelled serum protein antigens. Small numbers of cells laden with antigen (approximately 1/2000 to 1/5000) were identified in TDL and blood by autoradiography or fluorescence microscopy. The antigen-laden (Ag-L) cells resembled macrophages in that a large number adhered to plastic, they phagocytosed bacteria or a particulate dye, were non-specific esterase positive, radioresistant and could take up more than one antigen at one time in vivo. Surface phenotyping using monoclonal antibodies against rat cell markers established that Ag-L cells did not express Ia determinants. The results suggest the existence of a subpopulation of macrophage-related cells that may be involved in the transport of antigen and in stimulation of antibody responses.