Lewis rats were given a single i.v. injection of soluble immune complexes containing human serum albumin (HSA) and rabbit anti-HSA antibodies, prepared in antigen excess. This resulted in localization of HSA and rabbit gamma globulin (RGG) in glomerular mesangial regions without producing definite histologic changes. 24 h after the injection of immune complexes, groups of these rats received lymph node cells or T-cell preparations from syngeneic donors sensitized to RGG, HSA, or ovalbumin; another group received no cells. All of these groups and a group of normal control rats were given injections of [3H]thymidine at 18, 27, and 44 h. The animals were killed 48 h after the time of cell transfer. In histologic sections, glomerular abnormalities were found only in some of the animals that had received immune complexes and lymph node cells or T-cell populations from donors sensitized to HSA or RGG; the lesions were characterized by focal and segmental increase in cells in mesangial regions. Autoradiographs revealed significantly greater numbers of labeled cells in mesangial regions and glomerular capillaries in the groups that had received immune complexes and cells from HSA- or RGG-sensitized donors than in any of the other groups. Electronmicroscopic studies suggested that the increase in cellularity in mesangial regions resulted from an influx of mononuclear phagocytes. The findings indicate that cell-mediated reactions can be initiated by the interaction between sensitized T lymphocytes and antigens present in immune complexes within mesangial regions.