The locations of steroid binding sites with specificities for 17β‐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 17β‐oestradiol were specific, with3H‐17β‐oestradiol being displayed by unlabelled 17β‐oestradiol, but less readily displaced by either progesterone or testosterone. The binding sites for testosterone were less specific than those for 17β‐oestradiol or those for progesterone. Both 17β‐oestradiol and progesterone were good competitors for testosterone binding sites, while the binding of3H‐progesterone was easily displaced by 17β‐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 17β‐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 mot