To address the question of whether prechoriogenic follicles of the silkmoth Bombyx mori have the capacity to enter choriogenesis in organ culture and define the stage at which choriogenesis becomes established as a follicle-autonomous program, we have cultured immature ovarioles dissected from developing pupae and examined the protein synthetic profiles of follicular cells of individual follicles at the end of the culture period. The protein synthetic profiles of the cultured follicles were also correlated with corresponding profiles of chorion mRNA accumulation. Our results demonstrate that the last 17 (+/- 2) vitellogenic follicles of Day 5 to 7 pupae are capable of initiating choriogenesis in organ culture. The earliest vitellogenic stage to enter choriogenesis in vitro does so after 34 (+/- 4) hr in culture and follicles entering choriogenesis in vitro are capable of proceeding through all choriogenic stages at a speed comparable to that occurring in vivo. Therefore, once the choriogenic program becomes established in follicular cells, it can be implemented autonomously in the absence of extrafollicular factors. Earlier vitellogenic stages lack this capacity, presumably because they require additional hemolymph factors to establish the choriogenic potential. Our results demonstrate that the choriogenic potential of cultured vitellogenic follicles cannot be influenced by addition of 20-hydroxyecdysone to the culture medium.