Vitelline coats (VCs) of Phallusia mammillata were isolated and purified following homogenization of live eggs in order to investigate the molecular basis of sperm-egg recognition. Clean VCs were partly solubilized by sonication in H2O and the soluble fraction (SFVC), derived from the outer surface of VCs, was used for further characterization. Electrophoretic analyses of radioiodinated VCs revealed that SFVC consists of two major glycoprotein components with apparent average Mr's of 450,000 and 180,000, respectively. The 450,000 Mr component is composed of several charge isomers, whereas the 180,000 Mr component is supposed to consist of two oligomers, both with acidic pI, held together by a disulfide linkage(s). Each of the two components possesses WGA-binding sites as shown by transblotting followed by WGA-peroxidase treatment. The amino acid composition of SFVC was determined after acid hydrolysis and its carbohydrate composition was analyzed and quantified by GLC. GlcNAc and GalNAc were found to predominate with 86% by weight of total sugar content and fucose, mannose, and glucose accounted for the remaining 14%. The susceptibility of SFVC to enzymatic (N-glycosidase F) and chemical (TFMS) deglycosylation as well as to protease (trypsin and chymotrypsin) digestion was investigated. Furthermore, sperm receptor activity of SFVC was tested in a fertilization assay. The fertilization rate decreased in a concentration-dependent manner when sperm were preincubated with SFVC. Additionally, sperm treated with SFVC showed binding for FITC-WGA or WGA-gold at the apical portion of the sperm head. Therefore, we strongly assume that one or both of the identified glycoprotein macromolecules of SFVC are involved in sperm-egg recognition.