Despite the prominent role of IgA, particularly IgA₁, in the pathogenesis of IgA nephropathy (IgAN), the precise role of this molecule in the process remains unclear. Four biotin-conjugated lectins in sandwich-type enzyme-linked immunosorbent assays were devised to determine the glycosylation profiles of total IgA and its subclasses. We took advantage of differential binding properties of these lectins to sugar residues to dissect the oligosaccharide chainsO-linked to the hinge and thoseN-linked to the Fc region of total IgA and IgA subclasses in 47 patients with IgAN and an equal number of controls. The proportion of sialylated IgA₁ was higher in patients compared with controls (p<0.02), whereas IgA₂ in patients appeared less well sialylated. A reduction of galactose in pathological IgA as detected by RCA-I became significant after treatment of the molecule with neuraminidase (p<0.01). Defective galactosylation was also observed for patient IgA₁ when it was probed with ECL, a lectin that has a specificity for Gal 1,4N-acetylglucosamine groupings onN-linked oligosaccharides. The RCA and ECL results, therefore, suggest that increased sialylation on the IgA₁ is onO-linked oligosaccharides in the hinge region. This was partly confirmed by a small increase in the binding of PNA to IgA₁ from the patient group. This lectin binds preferentially to Gal 1,3N-acetylgalactosamine groups that are found onO-linked oligosaccharides.