The decapeptide luteinizing hormone releasing factor (LH-RF) has recently been isolated, sequenced and synthesized (Schally, Arimura, Kastin, Matsuo, Baba, Redding, Nair, Debeljuk & White, 1971). This has made possible the development of radioimmunoassays for this factor enabling it to be measured in biological fluids in vivo and in vitro. Two milligrammes of the synthetic decapeptide (Hoechst) were conjugated to 2 mg bovine serum albumin (BSA) using 75 mg 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide in 0·25 ml water (Goodfriend, Levine & Fasman, 1964). After dialysis overnight, 1 mg of the conjugate in 2 ml water, emulsified in Freund's complete adjuvant, was injected into 20 intradermal sites in a white New Zealand rabbit. A blood sample was obtained 8 weeks after this primary immunization and the assay developed using this antiserum. LH-RF (0·1–1 μg) was iodinated with 125I (0·5–1 mCi) by the chloramine-T technique (5 μg of chloramine-T), specific activities between 100 and 500 μCi/μg being obtained.