Proteins of the epididymal lumen have been the subject of investigation recently because they are important components of the environment in which spermatozoa mature. Many of these proteins have been shown to be secreted by the epididymal epithehum of laboratory and domestic animals as well as man. Protein secretion in the epididymis of the mouse, whose small body size has permitted radioactive labeling in vivo as well as in vitro for radioautographic studies of the secretory process The intracellular pathway for protein secretion has been mapped, and the various segments of the epididymis have been compared with regard to the pathway and the kinetics of secretion. However, the radioautographic techniques used in these studies did not reveal the nature of the proteins involved, except that some were glycoproteins since they were labeled with a radioactive sugar.