Antibodies to rabbit acrosome stabilizing factor (ASF) were raised in mice and proved monospecific on Western electroblots . Anti-ASF was utilized to immunolabel tissue sections of male reproductive tract organs. Staining of principal cell cytoplasm was observed primarily in the corpus epididymidis (Regions 6 and 7), and secondarily in the cytoplasm of principal cells of the distal cauda epididymidis (Region 8b ) and the columnar cells of the vas deferens epithelium. The microvilli of principal cells in the proximal cauda epididymidis (Region 8a ) were densely stained. Spermatozoa appeared uniformly stained within the lumen of the corpus epididymidis and staining intensity increased distally. The Golgi region of corpus principal cells was not stained, nor were other cell types in this region. Testis, caput epididymidis, and accessory sex organs were not stained. Synthesis of ASF by corpus epididymidis was shown by immunoprecipitation of radiolabeled ASF from organ cultures of specific epididymal segments. Scant amounts of synthesis were also detected in the cauda epididymidis and vas deferens. The large subunit of ASF, immunoprecipitated from the corpus epididymidis, is 2000-4000 daltons larger than the large subunit of ASF from more distal regions of the reproductive tract, suggesting modification of this component.