Renal immunomicroscopy using enzyme labeled reagents has been shown to be a reliable method for the identification of immunoglobulins and complement in the routine evaluation of glomerular disease. However, the potential carcinogenicity of benzidine derivatives used in the procedure represents a major disadvantage of the technique. With a series of 55 renal biopsy specimens evaluated by light microscopy, immunofluorescence, and electron microscopy from patients with a variety of renal diseases, a study was done comparing aminoethylcarbazole and the Hanker-Yates reagent (p-phenylenediamine and pyrocatechol), chromogens chemically unrelated to benzidine. Aminoethylcarbazole was not suitable for renal immunomicroscopy, since in cases of antiglomerular basement membrane disease the color reaction product was finely granular at high magnification. Specimens immunostained with the Hanker-Yates reagent yielded permanent water insoluble reaction products and immunomicroscopic patterns identical to the results observed with immunofluorescence in all cases. To our knowledge, no carcinogenic properties have been identified for p-phenylenediamine or pyrocatechol. The Hanker-Yates reagent may be used routinely for renal enzyme immunomicroscopic studies with no currently identified carcinogenic hazard to laboratory personnel.