Currently there is increasing interest in the localization and accurate determination of the composition of protein deposits involved in the pathogenesis of human glomerulonephritis using immunoelectron microscopy. Previous studies have largely relied on capricious immunoperoxidase techniques applying both the preembedding and postembedding methods to renal tissue. Both methods identify a single antigen, but preservation of ultrastructure is suboptimal, and there is the potential problem of imprecise localization of antigen due to diffusion of the peroxidase reaction product. A less demanding method of ultrastructural antigen identification for renal tissue has been employed with success in clearly localizing glomerular deposits of IgA. The technique involves fixation of tissue in periodate-lysine-paraformaldehyde, embedding in L. R. White resin, and labeling by an indirect immunologic reaction using protein A-gold as an electron-dense marker. This technique provides an accurate marker for the localization of glomerular antigen while retaining optimal tissue morphology, and we predict that this method will have wide application in human and experimental renal disease.