The asparagine-linked oligosaccharides of human C3 were characterized. The C3 oligosaccharides were released by endo-beta-N-acetylglucosaminidase H and were analysed by lectin affinity chromatography and h.p.l.c. The released oligosaccharides bound tightly to concanavalin A-Sepharose and were not retained by agarose-bound wheat-germ agglutinin, indicating that they were only of high-mannose type. Two major oligosaccharide structures were separated from both the alpha- and beta-chains of C3 by h.p.l.c. on Micropak AX-5, calibrated with high-mannose-type oligosaccharides of known structures. The oligosaccharide structures on the alpha-chain have the compositions (Man)9(GlcNAc)2-Asn and (Man)8 (GlcNAc)2-Asn, and those on the beta-chain have the compositions (Man)6(GlcNAc)2-Asn and (Man)5(GlcNAc)2-Asn.