A radiochemical method was adopted to analyze the in vitro products of the corpus allatum (CA) ofPlautia stali.The radiolabel derived from3H-methionine added to the incubation medium was incorporated and released by CA as two main radiolabelled products. They showedRfvalues of about 0.3 and 0.5, respectively, in the thin layer chromatography (TLC) system used. The release of these products was shown to be CA-specific since in control incubations using the brain, midgut, aorta and flight muscle, virtually no release of these products was observed. The locations where these main products migrated on the TLC did not coincide with spots of synthetic standards of JH I-III or JHB3, a JH found in higher Diptera. An addition of precursors of JH III, farnesoic acid or farnesol stimulated the CA to biosynthesize the products with anRfvalue of 0.5 up to about 10-fold, suggesting that the product in question may have a sesquiterpenoid skeleton similar to JH III. Topical applications of the hexane extracts of the medium in which the CA had been incubated exerted a juvenilizing, metamorphosis-inhibiting effect on final instar nymphs in a dose-dependent fashion. The nymphs treated with the hexane extracts at a high dose moulted to intermediates with reduced forewings and scutellum, as well as nymphs implanted with the CA from reproductively active females. Based on this juvenilizing effect found in the hexane extracts, the JH-active fraction was determined after TLC separation. This assay indicated that the products found at anRfvalue of about 0.5 was JH-active. These results suggest the presence of a new JH different from any known JHs inP. stali.