Three patients presented with renal or more diffuse tissue deposits of a nonamyloid material reactive with anti-kappa antibody by immunofluorescence. All patients had progressive renal failure with the nephrotic syndrome and extensive tubular basement membrane deposits. Glomerular lesions were conspicuous but heterogeneous. One patient also had hepatic deposits with peliosis at histopathologic examination. An underlying lymphoplasmacytic disorder was found in all patients: multiple myeloma in one, pleomorphic lymphoplasmacytic malignancy analogous to Waldenström's macroglobulinemia in one and bone marrow monoclonal plasmacytosis without overt myeloma in one. Biosynthesis experiments in two cases showed production of abnormal kappa chains which were not detected in appreciable amounts in serum and urine. These light chains had an aberrant size (abnormally short or large), their apparent molecular weight was larger in secretion than in cytoplasmic extracts (suggesting their glycosylation) and they were secreted as polymers. These results suggest a causal relationship between production of abnormal light chains and tissue deposition.