Oncomodulin is an acidic calcium-binding protein of 108 amino acids which has been detected only in tumors. The complete amino acid sequence of rat oncomodulin has been established, and the N-terminus shown to be N-acetylserine. Homology of the oncomodulin sequence with members of the troponin C superfamily of calcium-binding proteins was demonstrated, and putative calcium-binding domains were located. A further degree of homology was established between oncomodulin and the parvalbumins, including rat parvalbumin. Oncomodulin was therefore considered to be a parvalbumin-like calcium-binding protein.