We have developed a radioimmunoassay for insect juvenile hormones. The C18 hormone (JH I) was first converted into diol by opening the 10, 11-epoxy ring. Diol was then succinylated and coupled to human serum albumin to make it immunogenic. High titer antisera were obtained from immunized rabbits. Succinyl juvenile hormone was also coupled to the peptide glycyltyrosine and iodinated so as to form a water-soluble 125I-labelled analogue well recognized by antibodies (60% bound by the 1/2000 000 dilution of antiserum routinely used in radioimmunoassay). Standards and biological samples were treated with acidic dioxane in order to convert each hormone into its corresponding diol. In this way, the sensitivity threshold of the radioimmunoassay was under 0.015 pmol. All three diols were equally recognized by the antibodies. Hormones JH I, JH II and JH III could be assayed separately as diols after thin-layer chromatography or high-pressure liquid chromatography purification of the biological samples. This method was used to determine physiological levels of juvenile hormones in the haemolymph of several insects at different development stages including embryos and in corpora allata cultures.