We have previously characterized a subpopulation of cells in the rat glomerular mesangium that are phagocytic, display Ia antigens, and stimulate lymphocytes. We now report that total-body irradiation induces a progressive loss of the Ia+ mesangial cells, which is complete by 3 days. Experiments utilizing kidney shielding and in vitro exposure to irradiation have established that the effect is not mediated via radiation sensitivity of the mesangial phagocyte. Transplantation of syngeneic bone marrow cells results in a restoration of Ia+ mesangial cells in 3 weeks, establishing their extrarenal origin. Employing monoclonal antibodies to rat leukocyte common (LC) antigen, we have quantified a larger population of mesangial phagocytes; the Ia+ cells are a subset of the glomerular LC+ cells. In tissue culture, anti-LC antibody labels 100% of the phagocytic cells obtained from perfused glomeruli. Ultrastructural analysis, utilizing immunoperoxidase techniques, has localized the LC+ cell to the glomerular mesangium.