The studies described deal largely with the properties of a peroxidase in rat uterine fluid and human cervical mucus with particular regard to its toxic effect on bacteria, fungi, viruses and spermatozoa when combined with H202 and an appropriate oxidizable cofactor such as iodide, bromide or thiocyanate ions. The peroxidase may be either leukocytic in origin (eosinophil, neutrophil) or it may be secreted by epithelial cells lining the cavity. Such a system may exert a considerable influence on the control of the microbial flora in the vaginal and cervical canal and on the control of sperm motility and thus fertilization. Abnormalities in this system will be sought in patients with vaginal or cervical infections or with infertility problems. The degradation of estrogens by normal and abnormal leukocytes and the interaction between spermatozoa and intact leukocytes will be investigated.