The locations of steroid binding sites with specificities for 17 beta-oestradiol, progesterone and testosterone have been examined on human spermatozoa by autoradiography. The binding sites were not evenly distributed over the entire surface of human spermatozoa. They were more densely located on the midpiece of the tail, less on the head and neck, and least on the principal- and end pieces of the tail. The binding sites for 17 beta-oestradiol were specific, with 3H-17 beta-oestradiol being displayed by unlabelled 17 beta-oestradiol, but less readily displaced by either progesterone or testosterone. The binding sites for testosterone were less specific than those for 17 beta-oestradiol or those for progesterone. Both 17 beta-oestradiol and progesterone were good competitors for testosterone binding sites, while the binding of 3H-progesterone was easily displaced by 17 beta-oestradiol, but less readily displayed by testosterone. Since there was preferential binding of these steroids to the mid-piece, and strong competition for binding between 17 beta-oestradiol and progesterone, these studies appear to provide information on the sites of action for the reported effects of these and related steroids on spermatozoal motility, migration and metabolism.