In the human and subhuman primates the uterine cervix plays an important role in the reproductive process by its permissive and inhibitory action on sperm migration from the vaginal pool into the cervical canal, the uterine cavity and the fallopian tube, the site of gamete unification and fertilization. This is accomplished through physico-chemical (amount, clarity, viscosity, pH, electrolyte composition, etc.) alteration of the cervical mucus in response to the circulating sex steroids. In an ovulatory cycle, shortly prior to and at the time of ovulation the cervical mucus becomes most receptive to the spermatozoa whereas at other times, specifically following ovulation, it becomes hostile to the spermatozoa and virtually impenetrable. This unique property of the cervical mucus may, in addition to the presently available techniques (diaphragm, cervical cap and intracervical devices), allow identification of such potential contraceptive modalities as: pH modifier - changing the pH of the cervical mucus from alkaline to acid around the time of ovulation; Electrolyte modifier - changing electrolyte composition of the cervical mucus to produce a mesh impenetrable to spermatozoa. Finally, development of a temporary localized tissue-fixed immune antibody to spermatozoa in the cervical mucus is within the realm of reality and deserves the necessary attention.