Modifications of the human sperm surface during incubation in capacitating conditions were studied by radiolabeling terminal sialic acid residues of cell surface glycoconjugates using the sodium metaperiodate/sodium tritiated borohydride method. During in vitro capacitation, sialyglycoconjugates were released from the human sperm surface according to well reproducible kinetics. This release could be inhibited by the presence of seminal plasma in the capacitation buffer. Two principal size classes of sialyglycoconjugates were detected in the capacitation medium and analyzed by gel filtration chromatography and SDS-PAGE. The smaller class was characterized by glycopeptides less than 5,000 Da, whereas the larger class was characterized by two sialyglycoproteins of approximately 15,000-16,000 and 22,000-23,000 Mr. The role of human albumin, a key component of the capacitation buffer, in the removal of these molecules from the sperm surface was studied in light of its constant association with large amounts of released material.