Three new solvent systems, pyridine: benzene (2.5:20, v/v), methanol:carbon tetrachloride (1:20, v/v) and acetone:dichloromethane (0.3:8, v/v), for the resolution of a mixture of 18 PTH amino acids have been reported. Using these solvent systems, various combinations of PTH amino acids which had previously posed resolution problems have been resolved and identified.