The primary structure of the calf thymus non-histone chromosomal protein HMG-17 has been determined. The sequence was determined mainly from data provided by the peptides obtained by cleavage with staphylococcal protease. Additional information was obtained from peptides produced by cleavage with trypsin and alpha-protease (from Crotalus atrox venom) and by partial acid hydrolysis. The protein is 89 amino acid residues in length, and has molecular weight of 9247. The N-terminal two-thirds of the molecule is highly basic, 22 of the first 58 residues being lysine or arginine, whereas only seven residues are aspartic or glutamic acid residues. In contrast, the C-terminal region of the molecule has an overall negative charge, only four of the last 31 residues being basic, whereas seven aspartic and glutamic acid residues are present. The protein is also characterised by a region of high density of proline residues, there being six proline residues between residues 31 and 40. A region of 19 residues sequence similarity with the trout-specific histone, H6, is noted together with some smaller regions of sequence similarity with histones H1 and H5.