Heymann's nephritis can be induced in rats by injection of antibodies (Ab) against the major nephritogenic antigen (Ag) (gp330). We previously reported on a 90-kD tubular-glomerular glycoprotein (gp90) having enzymatic activity (dipeptidyl-peptidase IV) which, after antibody interaction, induces a transient Heymann-like immunofluorescence pattern. In the present study we have analysed the ultrastructural features of the renal immune reaction occurring in rats after injection of a monoclonal antibody to gp90, revealed by a pre-embedding immunogold staining (5-nm particles). In the early stage (10-60 min) gold granules were found along the glomerular capillary walls mainly in the endothelial region. During the intermediate stage (24-48 h) the brush border microvilli were diffusely labelled. In the late stage (1-2 weeks) gold particles were still noticed at the tubular level. This work suggests that the immune reaction in Heymann's nephritis also occurs on the endothelial side of the glomerular capillary walls, thus facilitating the passage of epithelial-specific antibody. The tubular kinetics confirms that gp90 may play a role in the metabolic activity of tubular proximal cells. Because this antigen has been demonstrated in human kidney as well, it may be relevant to some membranous or other human nephropathies.