Previous studies have shown that the larva of Chelonus inanitus plays an essential role in inducing the precocious onset of metamorphosis in polydnavirus/venom-containing Spodoptera littoralis, and that this effect might be due to proteins released by the parasitoid larva into the host. Here we show that a 212kD protein is the predominant protein released in vitro by late first instar parasitoids. This protein does not accumulate anywhere in the parasitoid body. Polyclonal antibodies specific for the 212kD protein were produced and immunoblot analyses revealed that this protein begins to be released by parasitoid larvae when the precocious onset of metamorphosis is induced in the host. Amino acid sequences of the N-terminus and of internal fragments of the 212kD protein were determined and glycoprotein detection showed that this protein is glycosylated. The availability of specific antibodies and the amino acid sequence information should now allow us to clone and express the 212kD protein and to directly study its functional significance. none