MRC Radioimmunoassay Team, 2 Forrest Road, Edinburgh, EH1 2QW (Received 9 May 1974) Luteinizing hormone releasing hormone (LH-RH) stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary. Radioimmunoassays have been developed in various laboratories to measure this decapeptide. In studies on man, Jeffcoate, Greenwood & Holland (1974) found that 0·75–2·8% of an injected dose of LH-RH was excreted in the urine and that the excretion rate of endogenous LH-RH was about 1·4 μg/day, although the apparent daily excretion rate was reduced 50-fold using a more specific antiserum (Jeffcoate et al. 1974, note added in proof). A radioimmunoassay for LH-RH has been established using antiserum raised against a bovine serum albumin–LH-RH conjugate (Jeffcoate, Fraser, Gunn & Holland, 1973), given by Mr H. M. Fraser. After iodination of synthetic LH-RH (Hoechst), labelled peptide was separated from free iodide by gel filtration on Sephadex G-10. Material