Urinary stones with amyloid structure, obtained from uremic patients, were analyzed according to molecular weight, amino acid sequence, and antigenic content. A major protein of approximately 7 kD, designated AB protein, was isolated by size exclusion using HPLC in 60% formic acid. AB protein reacted in immunodiffusion only with an antiserum to beta 2-microglobulin, with beta 2m spurring over AB protein. N-terminal amino acid sequence analysis defined two fragments homologous to beta 2m. One fragment commenced with Ile at position 7 and the other with Ser at position 20, with a cleavage point subsequent to a lysyl residue in both. It is concluded that beta 2m is a precursor of urinary amyloid stones and intratubular concretions of patients with preterminal and terminal renal failure; limited proteolysis is involved in AB amyloid generation.