Abstract: Serial assay of guaiacol peroxidase (G-P.sub.x) in the cervical mucus of human females during the menstrual cycle demonstrated that G-P.sub.x levels decrease 20- to 100-fold in the middle of the cycle relative to levels at other times in the cycle. Cervical mucus can be sampled either at the external cervical os or in the vaginal canal. The test for assaying G-P.sub.x includes extracting the sample with a Ca.sup.++ -containing buffer solution at a slightly alkaline pH, and thereafter adding guaiacol and a peroxide. Decreasing levels of G-P.sub.x are indicated by a color change from deep red to pink to yellow. The decrease was found to coincide with the peak of plasma estrogens that normally precedes ovulation. Accordingly, the test provides a reliable prediction of impending ovulation and is therefore useful in the practice of natural fertility control.