A test for glucose in mucus from the cervix can be used to indicate the escape of the ovum from the graafian follicle. The glucose fertility test involves paper tape impregnated with glucose oxidase, which catalyzes the oxidation of glucose to gluconic acid. Hydrogen peroxide, a by-product of the reaction, with a peroxidase plus an oxidizable dye changes the tape's pink color to blue. Tested prior to undergoing abdominal operations, 35 women had positive glucose reactions; all were observed during their operations to have ruptured graafian follicles or early activity of the corpora lutea. This relationship was unequivocally proved by careful dating of simultaneous endometrial and ovarian biopsies obtained by culdotomy. Natural child spacing by positive or nega tive family planning can therefore be carried out inexpensively by intelligent continence on the basis of this evidence of ovulation.