Washed ejaculated human spermatozoa were surface labelled with 125I, using solid phase (iodogen) or enzymic (lactoperoxidase) methods, while membrane components possessing terminal galactose or galactosamine residues were labelled with the galactose oxidase-sodium [3H]borohydride technique. All three procedures revealed the presence of 2 major labelled surface components. The first comprised a broad band of radioactivity migrating just behind the ion front on SDS-PAGE, which could be extracted with chloroform and methanol, suggesting a lipid-like composition. The second fraction exhibited properties consistent with a major glycoprotein component of the human sperm plasma membrane, giving a peak of radioactivity with Mr = 20,000, within which a discrete doublet of bands (Mr = 17,000 and 19,000) could be resolved by autoradiography. A more detailed analysis of the labelled protein fraction after TCA precipitation revealed a number of other surface components, the major ones of which exhibited Mr values of 30,000, 45,000, 66,000, 115,000 and 160,000. Western blot analysis was then used to determine whether any of the surface components described above interacted with the gamma-globulin fraction of antisera obtained from patients exhibiting idiopathic autoimmunity against sperm antigens. Using a purified membrane preparation as the target, antibodies were detected against numerous high molecular weight bands with Mr values similar to the major components of the human sperm surface (35,000, 45,000, 66,000, 90,000 and 150,000). The nature of the antigens targeted by these antisera did not correlate with the ability of the latter to stimulate or suppress sperm-oocyte fusion.