Previous work suggested that the substitution of Asn for Ser at position 406 of the mu heavy chain of mouse IgM results in aberrant glycosylation at Asn402. In order to characterise the apparently abnormal glycosylation process more precisely, the mutant and wildtype mu chains were fragmented by cleavage with cyanogen bromide, and the resulting glycopeptides were analysed further. Measurements of lectin binding specificity as well as glycosidase sensitivity suggest that the oligosaccharide at Asn402 of wildtype mu is a hybrid type which does not contain terminal alpha(2-6) or alpha(2-3) linked sialic acid. By contrast, the corresponding oligosaccharide on Asn402 of mutant mu is complex and contains terminal sialic acid linked alpha(2-6) to galactose. The structural features for specifying the abnormal glycosylation are present in monomeric mutant IgM.