Infertile couples undergoing routine investigation for infertility were randomly selected for the study. A quantitative method using weighed swabs was found satisfactory for the study of microbial flora in the midcycle cervical mucus. In 6 of the 20 women, the midcycle cervical mucus was hostile in the sperm cervical mucus penetration tests. The total bacterial counts and the number of species (average, 2.5) isolated from the hostile mucus were significantly greater than those from the receptive mucus (average, 1.3). Only normal vaginal/cervical flora was isolated. Ureaplasma urealyticum was found more frequently in patients undergoing clomiphene citrate therapy (6/10 versus 0/9, P less than 0.03, Fischer's Exact test).