The imaginal discs of Sarcophaga were found not to develop normally in the presence of galactose, a hapten sugar of Sarcophaga lectin, or anti-Sarcophaga lectin antibody. Wing and leg discs cultured with these substances became morphologically abnormal and no imaginal discs reached the stage of terminal differentiation, even in the presence of 20-hydroxyecdysone. The development of the imaginal discs was shown to be autonomously regulated in an autocrine manner by Sarcophaga lectin; namely Sarcophaga lectin was secreted by the imaginal discs in the presence of 20-hydroxyecdysone, and the stimulus of self-induced Sarcophaga lectin seemed to be indispensable for further development of the imaginal discs. Sarcophaga lectin was originally found as a defense protein, but these results show that it plays independent roles in both defense and development.