The immunochemical nature of the light chains of the mesangial IgA deposits were studied in 13 IgA nephritic patients with only IgA isotype in their renal biopsies. Direct immunofluorescence (IF) using rabbit and goat antihuman monospecific antibodies demonstrated lambda light chain IF in all 13 biopsies (100%). Kappa light chain IF was detected in ten renal biopsies by the rabbit antihuman antisera and in eight renal biopsies by the goat antihuman antisera. Monoclonal antibodies studies showed lambda IF in all 13 biopsies and kappa IF in eight biopsies. Twelve biopsies showed a predominance of lambda light chain IF staining in the mesangial deposits. Identical IF patterns were observed when double IF technique with rhodamine-conjugated antihuman alpha heavy chain antiserum and fluorescein-conjugated antihuman light chain antisera were employed, and these findings confirmed that the light chains IF arose directly from the IgA deposits. These IgA nephritic patients had a significantly higher serum concentration of lambda light chain-IgA and a distinct alteration of kappa/lambda ratio of IgA compared with healthy controls. Our results support our preliminary observation that mesangial IgA deposits in IgA nephropathy consist mostly of IgA with lambda light chain.