A test was set up to analyze the direct binding of serum IgA to the lectin jacalin. Under the testing conditions, jacalin bound to both IgA subclasses and reacted similarly with monomeric and polymeric IgA. A jacalin index was defined to quantify serum IgA binding to this lectin. The jacalin index appeared significantly lower in IgA nephropathy than in controls. This may be related to abnormal IgA glycosylation, which could explain, at least in part, the mesangial deposition responsible for the renal disease.