Astins A, B and C, three antitumour chlorine-containing cyclic pentapeptides were isolated from Aster tataricus and their structures were elucidated by spectroscopic and chemical evidences. Astins A and B, each containing an allo threonine at residues 5 and 2, respectively, and both of which were isomers, showed more potent antitumour activity than astin C. Conformational analysis of astin B in solid state was conducted by X-ray crystallographic analysis, showing a cis configuration in one of the amide bonds.