During the survey of novel antitumor compounds from medicinal plants, n-butanol extract from Aster tataricus showed potent antitumor activity. A. tataricus (Compositae) is known as a Chinese medicine containing several terpenoids and saponins and is also popular as a garden flower. Efforts at chromatographic purification of the n-butanol extract guided by antitumnor activity led to the isolation of nine cyclic pentapeptides and one acyclic peptide, named as astins A-J (1-10). The structures of a series of astins, containing one or two chlorine atoms, were elucidated by spectroscopic evidence, chemical degradation and chemical transformation. Conformational analysis of astin series was studied by spectro-scopic (NMR and X-ray analysis) and computational chemical methods (molecular dynamics and molecular mechanics calculation). These results indicated that the conformation in the solution state was, on the whole, homologous to that observed in the solid state. The difference between astin B. showing a cis configuration in a proline amide bond, and cyclochlorotine from penicillium islandicum, showing all trans amide configurations, was also discussed. To analyze the relationship between their structures and antitumor activities, various derivatives at Pro^1 and a thioamide derivative were synthesized. The presence of 3,4-dichloroproline was considered to play an important part to show antitumor activities. Further, the presence of an intramolecular hydrogen bond between Thr^2-OH and Ser^3-NH was necessitated to maintain the typical confor-mation appeared in astin B and was considered to relate the activity.