The Medical Unit, The Princess Margaret Hospital, Christchurch 2, New Zealand (Revised manuscript received 21 August 1978) The frequent clinical and research requirement for measurement of both plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) has prompted the development of a simultaneous radioimmunoassay for these two hormones. The considerable advantages of a simultaneous method include an economy of plasma volume, assay reagents, test-tubes and, more importantly, the time required for radioactive counting and performance of the assay by technical staff. The latter two factors comprise a significant proportion of radioimmunoassay operating costs. This report describes a simultaneous radioimmunoassay based on the use of 131I-labelled FSH, 125I-labelled LH, anti-FSH serum M93 6873 (a generous gift from Professor W. R. Butt, Birmingham), anti-human chorionic gonadotrophin (HCG) serum for LH measurement (Donald, 1972) and donkey anti-rabbit precipitating serum (Wellcome Reagents, U.K.) for separation of antibody-bound and free hormones. Pituitary gonadotrophin standard (LER 907)