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The brush-border membrane vesicles were isolated from the midgut epithelial cells of the last instar cabbage armyworm larvae (Mamestra brassicae). It was observed that 14C-UDP-Nacetylglucosamine (UDP-AGA) bound fast to the vesicles at the optimum pH of 7.0. Almost 9000 of the binding was inhibited by polyoxin D. Chitin was synthesized by the vesicles in vitro in the presence of chitodextrin as a primer. These facts suggested that much of the binding was based on the 14C-UDP-AGA-chitin synthetase binding, whose activity was stronger at 20C than 25C. On the other hand, the chitin synthesis by the vesicles progressed much faster at higher temperatures, indicating that the turnover of the binding would be faster at higher temperatures. The polyoxin D-treated vesicles retained the binding activity at the optimum pH 7.5, although UDP-ACA's binding activity became weak about one tenth of the original. The results led to a conclusion that the brush-border membrane vesicles had two substances bound to UDP-AGA, one would be chitin synthetase and the other a UDP-AGA transporter. However, the amount of UDP-AGA bound to the polyoxin D-treated vesicles was too small to conclude that the binding was based on the UDP-AGA transporter.