Vitelline-coat lysins from two species of marine mollusc of the genus Tegula, Tegula lischkei and Tegula sp., were purified and their properties compared with those of Tegula pfeifferi. Cross-reaction tests among these three species proved that the lysin action on the vitelline coat was species specific. Each of the lysins from these three species is a single, basic polypeptide with a molecular weight of 16000-17000 and an isoelectric point of pH 10.5. All of them share a common antibody recognition site(s) for the anti-T. pfeifferi lysin antibody. Their amino acid sequences were analyzed using intact lysins and peptides separated by reverse phase high-performance liquid chromatography after V8 proteinase digestion. The amino acid sequences of the N-terminus (Nos 1-66) from the three species showed 98% homology, and those of the C-terminus (Nos 91-118) showed 89% homology. It was concluded that the species specificity of the vitelline coat lysin appears to be determined by a sequence of 24 residues (Nos 67-90) in the middle region of the molecule.