SummaryA sensitive and specific radioimmunoassay technique for quantitating human luteinizing hormone (LH) is described. Purified human pituitary LH is used for the development of antibodies, for radioiodination and for standards. A good correlation was seen between biological and immunological potency estimates of a number of pituitary extracts varying greatly in their LH and FSH contents. This method, when applied to serum, revealed differences in levels between normal adults and postmenopausal women. Cyclic variation was noted in the normally menstruating woman and the presence of LH in the serum of prepubertal children was also confirmed.