Four unlabeled antibody immunocytochemical techniques, the "single bridge" (Avrameas S: Immunocytochemistry 6:825, 1969; Mason TE, Phifer RF, Spicer SS, Swallow RS, Dreskin RD: J Histochem Cytochem 17:190, 1969a; Sternberger LA, Cuculis JJ: 1969), the "single peroxidase-antiperoxidase (PAP)" (Sternberger LA, Hardy PH Jr, Cuculis JJ, Meyer HG: J Histochem Cytochem 18:315, 1970), the "double PAP" (Vacca LL, Rosario SL, Zimmerman EA, Tomashefsky P, Ng P-Y, Hsu KC: J Histochem Cytochem 23:208, 1975) and the "double bridge" (Ordronneau P, Petrusz P: Am J Anat 158:491, 1980) were compared at both the light and electron microscopic levels. The "double" procedures involved repeating incubations with the bridge antibody, in this case, sheep anti-rabbit gamma globulin, followed either by a second PAP step for the "double PAP" or a second anti-horseradish peroxidase step and a single incubation in horseradish peroxidase for the "double bridge." At both the light and electron microscopic levels the staining intensity was greater with the "double" techniques than with the "single" ones. This is probably due to amplification achieved with the second sheep anti-rabbit gamma globulin step, permitting an increase in the number of horseradish peroxidase molecules bound for each molecule of tissue-bound primary antibody. Also, the quality of the various commercial PAP preparations tested was variable. With the weaker ones the staining intensity could be increased by performing an incubation in fresh horseradish peroxidase after the PAP step. Finally, in electron microscopic studies, the reaction products formed in both the bridge and PAP procedures were identical in shape and size.