This study was designed to establish whether fluctuation in cervical mucus peroxidase concentration correlates with the cyclic pattern of the menstrual cycle hormones in the human female. Forty healthy, normal-cycling women between the ages of 19 and 29 years were chosen for the study. Blood samples and cervical mucus were collected on days 6, 9 through 15, and 17 through 19 of the menstrual cycle for three consecutive cycles in each volunteer. Blood estrogen, progesterone, follicle-stimulating hormone, and luteinizing hormone levels were quantitated by radioimmunoassay, and cervical mucus peroxidase concentrations were measured spectrophotometrically. The data showed that in the typical menstrual cycle the mucus peroxidase peak was reached immediately prior to the luteinizing hormone/follicle-stimulating hormone surge and coincided with the estrogen peak. Consistent data in three consecutive menstrual cycles in each volunteer led us to postulate that the mucus peroxidase peak during the menstrual cycle precedes the ovulatory period in the normal healthy woman.