Lewis rats were injected intravenously with rabbit anti-rat glomerular basement membrane (GBM) antisera in doses that were sufficient to cause glomerular fixation of rabbit gamma globulin (RGG) detectable by immunofluorescence, but which failed to induce histologically detectable lesions. 24 h later, groups of rats received lymph node cells or serum from syngeneic donors that had been immunized with either RGG or ovalbumin; they were injected with [3H]thymidine three times during the next 2 days, and sacrificed 48 or 96 h after transfer. Only the rats given anti-GBM antiserum plus lymph node cells from donors sensitized to RGG showed histological glomerular lesions, in the form of segmental hypercellularly and necrosis. Autoradiographs revealed the greatest number of labeled cells in glomeruli in the same group. In analogous experiments, it was shown that T-cell-enriched populations could induce hypercellular glomerular reactions. On the basis of electronmicroscopic and autoradiographic observations, it appears that the glomerular hypercellularity resulted from both infiltration of mononuclear cells and proliferation of endothelial cells. The findings indicate that interaction of specifically sensitized lymphocytes with glomerular-bound antigen can induce a cell-mediated (delayed-type) reaction in glomeruli.