Recent studies have provided evidence for the involvement of macrophages (m phi) in various types of human and experimental glomerular disease. The aim of the present study was to examine the effect of m phi depletion on glomerular injury after 3/4 renal ablation in the rat. This" remnant kidney" model is a widely used experimental model of focal and segmental glomerulosclerosis. Sustained glomerular m phi depletion was induced in remnant kidney rats by a regimen of sublethal triple systemic X-irradiation with shielding of the kidney remnants. Groups of 8 X-irradiated and 8 non-irradiated rats were studied at 5, 9, and 13 weeks after renal ablation. X-irradiated rats showed severe peripheral blood leukopenia at 5 and 9 weeks which had normalized at 13 weeks. The number of remnant glomerular m phi (immunohistochemistry with the monoclonal antibody ED1) in X-irradiated rats at 5 weeks was significantly lower when compared to non-irradiated remnant kidney rats. A rebound effect occurred at 9 and 13 weeks with increased m phi in remnant glomeruli of X-irradiated rats. Light microscopic examination disclosed significantly lower semiquantitative scores for mesangial cellularity and mesangial matrix expansion in remnant glomeruli of X-irradiated rats at 5 weeks when compared to non-irradiated remnant kidney rats. Mesangial matrix expansion had increased in X-irradiated rats at 9 and 13 weeks after ablation coincident with elevated glomerular m phi at these intervals. Multiple linear regression analysis indicated a highly significant contribution of m phi to the best fitting regression model predicting mesangial matrix expansion (multiple r2= 0.81). In conclusion, these data provide evidence for a contributory role of m phi in the evolution of glomerular injury in the rat after renal ablation.