Amyloidosis is characterized by deposition of protein fibrils in various tissues. The wide variety of sequences of both amyloidogenic and non-amyloidogenic immunoglobulin light chains makes them a unique tool for addressing the importance of primary structure in the formation of insoluble fibrils. In this study, we have determined the primary structure of theκ I immunoglobulin light chain from both the urinary Bence Jones protein and the deposited amyloid fibrils of a patient (MH) with primary amyloidosis. The sequence identity of urinary-excreted and tissue-deposited light chains excluded biclonality and somatic mutations and confirmed that the light chain existed in both a soluble and an insoluble form. Several residues have been previously reported to be significantly associated with amyloidogenic κ chains. Many of these were found in the MH sequence, including Asp31, Asn45, Phe49, Gln55, His70, Asn/Gly93 and Pro/Val96, thereby supporting their potential role in fibrillogenesis. In addition, Asn20 and Pro60 of protein MH replaced the normally conserved Thr20 and Ser60. Asn20 was glycosylated in both the Bence Jones and the amyloid fibril protein MH. Cumulative effects of amyloid-associated residues and glycosylation might have rendered the immunoglobulin light chain MH prone to fibril formation.