Parasitization of newly ecdysed third, fourth, or fifth instar Manduca sexta larvae by the gregarious braconid wasp Cotesia congregata induces synthesis of new hemolymph proteins in the host. Analysis of hemolymph from parasitized and unparasitized control larvae using SDS gel electrophoresis showed that a major 33 kd band, plus several minor bands, were synthesized in parasitized but not control larvae; autoradiograms of proteins labeled in vivo for 1 hr with (35S)methionine indicated that synthesis of the 33 kd polypeptide began a few hours following oviposition by the wasp. Synthesis of the 33 kd parasitism-specific polypeptide was induced in unparasitized larvae by the injection of ovarian calyx fluid from adult female wasps: this fluid is known to contain two morphologically distinct types of virus particles that are normally injected into the host along with eggs during parasitization. Exposure of calyx fluid to psoralen in the presence of long-wave u.v. light destroyed its capacity to induce synthesis of the 33 kd protein, suggesting that synthesis of this polypeptide may be mediated by viral nucleic acid. none