The possibility of escape ovulation in women using a fluoroquinolone ofloxacin for antibacterial treatment at a dose of 200 mg twice daily for 7 days when taking a combined oral contraceptive (Microgynon, 150 micrograms levonorgestrel and 30 micrograms ethinyl estradiol) was studied in 20 women. By using a placebo-controlled, randomized, cross-over design 6 contraceptive pill cycles were followed. Follicle-stimulating hormone and estradiol concentrations were measured on tablet days 5-8, 19-21; progesterone was assayed on days 19-21 during treatment with ofloxacin or placebo. Ultrasonographical investigations for measurements of the number and diameter of ovarian follicles were performed on one of days 5-10, 11-17, 18-24, 25-28. No indications of ovulation during ofloxacin medication were detected. It is concluded that alternative contraceptive precautions should not be needed when ofloxacin is prescribed for patients taking oral contraceptives.