A 57-year-old Negro female patient is described with adult Fanconi syndrome, and marked x-light chain proteinuria over a 10-year period, without evidence of multiple myeloma. At necropsy, an unusual nodular form of amyloidosis was found. An antiserum to the x-chain was produced with a unique affinity for the glomerular amyloid of another patient with IgG-x myeloma. The urinary x-chain from both patients had an idiotypic antigenic site. The implications of these findings in relation to the pathogenesis of amyloid is discussed.