The effects of milk and yogurt on the bioavailability of ciprofloxacin were studied in seven healthy volunteers in a randomized crossover trial. After an overnight fast, 500 mg ciprofloxacin was given with 300 ml water, milk, or yogurt. Plasma ciprofloxacin concentrations were significantly (p less than 0.05) lower during the milk and yogurt phases from 1/2 to 10 hours; at 1/2 hour the concentration was reduced by 70% by milk and by 92% by yogurt. Milk reduced the peak plasma concentration by 36% (p less than 0.05) and yogurt by 47% (p less than 0.05). The extent of bioavailability, measured as the total area under the plasma concentration-time curve and 24-hour urinary excretion of ciprofloxacin, was reduced by 30% to 36% by milk and yogurt (p less than 0.05). We conclude that the absorption of ciprofloxacin can be reduced by concomitant ingestion of milk or yogurt. To avoid therapeutic failures in infections where the causative organism is only moderately susceptible, ingestion of large amounts of dairy products in liquid form with ciprofloxacin is not recommended.