The nature of the unique proteinaceous deposits in tissues in both systemic and localized amyloidosis has eluded investigative efforts for well over a century. Recent chemical and immunochemical evidence has demonstrated that in many cases of amyloidosis, the amyloid fibrils, which constitute one of the distinguishing features of the deposits, have as their major constituent a protein of immunoglobulin origin (1-3). These findings substantiate the suggestions of several investigators (4, 5), most notably those ofOsserman and co-workers (6-8). Amyloid fibrils of immunoglobu lin origin are most frequently demonstrated in patients with multiple myeloma or other plasma cell dyscrasia and usually, but not exclusively, in patients with the" primary"(idiopathic) form of the disease. In these cases the major protein of the amyloid fibril has been shown to be a homogeneous light polypeptide chain (L) and/or its amino-terminal variable (Vd fragment (3). Amino acid sequence analysis (1, 9, 10) and immunochemical investigation (2) have shown that these amyloid fibril proteins may be classified as either kappa-or lambda-type based on their chemical homology and/or sharing of their antigenic determinants with kappa or lambda Bence Jones proteins. In most cases further classification into subgroups, eg Vd, of these amyloid proteins has been possible (Table 1).