Vesicles were prepared from poly(sarcosine)-b-(L-Leu-Aib)(8) (SL16), and the membrane leakage of the encapsulated K+ was evaluated to be ca. 4% for 15 min at 30 degrees C. The osmotically induced leakage of the encapsulated 5(6)-carboxy fluorescein was negligible below 50 degrees C. The robustness of SL16 vesicle was considered to be due to the interdigitated structure of Leu side chains in the monolayer.