New polysialoglycoproteins, designated PSGP(On), were isolated from the fertilized and unfertilized eggs of the kokanee salmon, Oncorhynchus nerka adonis. The polysialylglycan chains consisting of alpha-2,8-linked O-acetylated poly(N-glycolylneuraminyl) chains have recently been characterized. We have now determined the complete amino acid sequence of the tandem-repeating units of PSGP(On) from the unfertilized eggs of kokanee salmon and found that the following two distinct forms are present in PSGP(On) in almost identical amounts: [formula: see text] and [formula: see text] where * denotes the O-glycosylation site and mean value of m, n = about 20. Upon fertilization these high-molecular-weight forms of PSGP(On) were proteolytically cleaved to the corresponding repeating units, low-molecular-weight PSGP(On), by the action of a specific protease (PSGPase) at the position two residues set C-terminally to the Pro residue and N-terminally to the Asp residue, i.e. -Pro-Ser-Xaa-Asp-: [formula: see text] and [formula: see text].