The immunochemical characterization of the light chains of the mesangial immunoglobulin A (IgA) deposits were studied in 45 patients with IgA nephropathy. Kappa and lambda light chains were detected with direct immunofluorescence (IF) method, using monospecific rabbit anti-human anti-kappa and anti-lambda anti-sera. The glomeruli of 42 renal biopsies studied were strongly positive for lambda light chain, while only 25 specimens were positive for kappa light chain. Sixty-five percent of the biopsies showed a predominance of lambda light chain IF staining in the mesangial deposits. This IF pattern is unique as compared with similar studies on renal biopsies from patients with systemic lupus erythematosus, idiopathic membranous nephropathy, and normal postmortem renal tissue. The results indicate that mesangial IgA deposits in IgA nephropathy consist mainly of IgA with lambda light chains despite the fact that the normal ratio of kappa to lambda light-chain-containing immunoglobulin in human serum is two to one.