The complete amino acid sequence of amyloid protein AND is presented. Amyloid fibrils were isolated from the spleen of patient AND, and the subunit protein was isolated from the fibrils after reduction and carboxymethylation. Sequence analysis of intact protein AND identified it as a kappa I immunoglobulin light chain. The complete sequence was determined from its tryptic peptides. The protein contained the entire variable region and the constant region to position 145 of the light chain. Several unique amino acid substitutions were found in protein AND compared to other kappa I proteins. The glycine, serine, arginine, threonine, alanine and arginine at positions 31, 45, 55, 76, 85 and 107, respectively, are reported for the first time in a kappa I protein. A number of uncommon amino acid substitutions were found in the framework regions in protein AND around the contact region of the dimer which may result in the molecule becoming more susceptible to fibril formation.