Fourteen healthy young women were studied through a control and a treatment menstrual cycle in two series of experiments. In the first series, they were given one of four doses of deglycosylated human chorionic gonadotropin (hCG) as a 24-hour infusion during the mid-luteal phase of the cycle. In these studies, there were no significant alterations of the length of the luteal phase of the treatment cycle, and there was no decrease in serum progesterone (P) during the infusion. In fact, serum P increased during the infusion. In the second series of studies, five subjects were given a 48-hour infusion of normal saline during the control cycle, and a 48-hour infusion of deglycosylated alpha-intact beta-hCG during the treatment cycle, both being administered during the mid-luteal phase. Treatment did not alter luteal phase duration and, again, increased serum P. It is concluded that deglycosylated preparations of hCG are not clinically useful as luteinizing hormone antagonists, probably because of residual agonist activity.