The effects on haemolymph ecdysteroid titre of a host noctuid moth in response to parasitism or injection of extracts of venom from the external parasitoid wasp, Eulophus pennicornis, were ascertained for the penultimate (fifth) instar larva of the tomato moth, Lacanobia oleracea. Unparasitized larvae showed a marked peak of ecdysteroid titre (85 ng/ml ecdysone equiv.) at approximately 48 h into the fifth stadium. This major ecdysteroid peak was eliminated, or its timing was significantly postponed (by at least 48-60 h), when the larvae were parasitized by E. pennicornis. Parasitized larvae failed to moult and eventually died. Saline-injected larvae showed a normal, but slightly delayed (ca. 12 h) major peak of ecdysteroid titre at 64-66 h (ca. 140 ng/ml) that followed a smaller peak (56 ng/ml) ca. 12-14 h earlier. As with parasitism, these ecdysteroid peaks were effectively eliminated, or significantly delayed, when larvae were injected with 0.7 gland equivalents of E. pennicornis venom. Venom-injected larvae failed to moult to the sixth instar, whereas saline-injected larvae moulted and developed normally. High-performance gel filtration chromatography was used for the initial purification of active venom components. Bioassays of the size-fractionated venom gland proteins by injection into fifth instar larvae showed that the suppression of moulting effect was confined to just two fractions (out of 12) with apparent M r between 43,000 and 158,000.