After lactoperoxidase-catalyzed iodination of either human sperm surface or human seminal plasma, a 72-76 kdalton component was resolved on SDS-7.5% polyacrylamide gels. The approximate molecular weight of this component was consistent with that of human lactoferrin, a major sperm-coating antigen. Immunoprecipitation of labeled sperm surface and of labeled seminal plasma with specific anti-human lactoferrin antibody identified lactoferrin as a constituent of this 72-76 kdalton component both on human sperm surface and in human seminal plasma. These data show that components which have been inferred to be shared because of similarities in biochemical characteristics also contain immunologically identical components.