**Abstract**

Glycophorins (GPs) in red blood cell (RBC) membranes of carp (*Cyprinus carpio* L.) exhibit bacteriostatic activity against various gram-negative and gram-positive bacteria including fish pathogens. This physiological property also exists in the GPs of yellow tail (*Seriola quinqueradiata*) and red sea bream (*Pagrus major*). Thus, we concluded that this antimicrobial activity is not confined to these teleost species but can be found in all fish. This bacteriostatic activity is caused by the sialo-oligosaccharide from these teleost GPs. Only the *N*-glycolylneuraminic acid (NeuGc) form of sialic acid was detected in the carp. Using NMR and GC–MS, we determined that the structure of the bacteriostatic sialo-oligosaccharide from carp was NeuGcα2→6 (Fucα1→4) (Glcα1→3) Galβ1→4GalNAc-ol. The bacteriostatic activity of this monosialyl-oligosaccharide is due to the property of the lectin receptor. It is supposed that some lectin-like proteins exist on the surface of gram-positive bacteria or the flagellum of gram-negative bacteria. Based on the electron microscope observations, teleost GPs containing the sialo-oligosaccharide are released from RBC membranes and then adsorbed onto the surface or the flagellum of invading bacteria in the blood.

**Keywords:** teleost, carp, yellow tail, red sea bream, red blood cell membranes, glycophorin, antibiotic activity, oligosaccharide, sialic acid
