A method which allows the highly sensitive and simple immunodetection of antigen-antibody complexes on nitrocellulose papers has been developed. The method is a modification of the procedure known as the "Nadi reaction" (oxidative coupling of 1-naphthol and N,N'-dimethyl-p-phenylendiamine) for histochemical purposes. Proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electrophoretically transferred to nitrocellulose membranes. Bound proteins were first reacted with a primary antibody and then with a horseradish peroxidase-labeled second antibody. The antigen-antibody complexes on the membranes were visualized with the oxidative coupling solution containing N,N'-dimethyl-p-phenylendiamine and 4-chloro-1-naphthol. Sensitivity was enhanced 4 to 16 times by the new method relative to that of the 4-chloro-1-naphthol method or the 3,3'-diaminobenzidine method.