Changes in the distribution of sialylglycoconjugates on the surface of uncapacitated and in-vitro capacitated human spermatozoa were studied by means of two sialic acid specific lectins [Maackia amurensis agglutinin and Sambucus nigra agglutinin). On the uncapacitated sperm surface, sialylglycoconjugates were found be localized from the post-acrosomal region of the sperm head to the tail middle piece, whereas after invitro capacitation these molecules were only found in a small area of the post-acrosomal region. The surface of capacitated human spermatozoa was also investigated by specifically radiolabelling its terminal sialic acid residues. A 20 kDa glycoprotein, which was partially purified by anion-exchange chromatography, was the main component of the sialylglycoconjugate pattern after in-vitro capacitation.