Luteinizing hormone (LH) levels measured with radioimmunoassay and time-resolved fluoroimmunoassay (FIA) in 274 serum samples correlated highly, with a linear correlation coefficient of 0.934. Through the analysis of serial samples from 43 women undergoing human menopausal gonadotropin stimulation for in vitro fertilization or gamete intrafallopian transfer and seven patients monitored in spontaneous menstrual cycles for receipt of frozen embryos, we demonstrated the utility of FIA in the detection of the LH surge. This LH assay technique, which involves no radioactive isotopes, should facilitate the monitoring of ovulation induction patients in the office/ambulatory setting.