Since predetermination of ovulation would be helpful in treatment of sterility, a quick, sensitive, and specific radioreceptor assay (RRA) for measurement of actual LH [luteinizing hormone] concentrations in human serum was developed. Using partially purified membrane receptors from bovine testes, this assay system enabled precise measurement of LH within 4 h. The detection limit of the method is 0.78 ng LER 960/ml serum. The method was used to detect ovulation during 4 observation cycles each in 11 women who were undergoing treatment for infertility, such as recommended intercourse or artificial insemination because of reduced fertility of their husbands. In all women ovulations could be predicted by LH surge and were verified by laparoscopy within 36 h. Insemination was carried out at the same time of the characteristic increase of LH values around mid-cycle. Pregnancy occurred in 3 women during the observation period.