The effect of a short course of ciprofloxacin on the oral microflora was examined in 16 healthy volunteers. Salivary specimens were collected before and after two 750 mg doses of ciprofloxacin administered 12 h apart. Salivary bacteria were identified to species level, and total quantitative colony counts were determined for each isolate. For all 16 subjects, treatment with ciprofloxacin led to a reduction in total bacterial colony counts; the mean inhibitory activity was 94.9% (range 79.6-99.9%). Both aerobic and anaerobic bacteria were inhibited in every case: 72.5-99.9% for aerobes, mean inhibitory activity 93.5%; 75.0-99.9% for anaerobes, mean inhibitory activity 96.9%. Total counts were reduced by two or more logarithms in 75% of individuals for aerobes and anaerobes. All classes of bacteria were inhibited, with mean inhibitory activities ranging from 91.3% to 100%. These findings, together with the high salivary levels and low toxicity of ciprofloxacin, suggest clinical studies are required to assess its suitability in preoperative prophylaxis of patients undergoing head and neck cancer surgery.