Abstract The origin of the protein which accumulates as large granules in the fat body of Drosophila melanogaster shortly before pupation, and the hormonal induction of this accumulation, were investigated. It was found that if 3H-labelled proteins and 14C-labelled amino acids are injected into larvae at the time of granule formation, the proteins are incorporated into granules but the free amino acids are not. Furthermore, injection of cycloheximide shortly before first appearance of the granules does not in any way interfere with their formation. It is therefore concluded that essentially all granule proteins are formed prior to appearance of the granules. On the basis both of injection of ecdysterone into larvae and of incubation of isolated fat bodies in vitro with and without hormone, it is concluded that ecdysone is a sufficient but not a necessary trigger for granule formation. The time at which granules appear suggests that ecdysone is not the normal inducer in this species. In the course of this work it was incidentally noticed that injection of ecdysterone into early third instar larvae causes a syndrome which resembles a partial and abortive larval-pupal apolysis. No puparium is formed and the larvae die within a few days.