The lipocalin now commonly called the epididymal retinoic acid-binding protein (ERABP) is a major secreted protein found in the lumen of the epididymis. The epididymis is an organ that consists of a single long convoluted tubule connected at one end to the testis and terminating at the vas deferens. Spermatozoa enter the tubule from the testis and, during transit, undergo a process of maturation that confers motility and the ability to fertilize an egg in vivo. The tubule has dramatic regional differences in the pattern of proteins secreted and the maturation process is dependent, in part, on these secreted proteins [1]. Thus, the presence of a carrier protein that is quite specific for retinoic acid suggests a role for the ligand either in sperm maturation or epididymal function.