In a prospective pharmacokinetic study the serum and tissue concentrations of ciprofloxacin (Bay O9867), a new carboxyquinolone antimicrobial agent were studied. 22 patients were given 300 mg ciprofloxacin i. v. before the operation (group A), and 19 patients were premedicated with 500 mg ciprofloxacin orally twice daily for three days followed by 300 mg i. v. preoperatively. Tissue samples weighing approximately 2 g were taken from the fallopian tubes, the ovaries, the fundus myometrium and the cervix. Ciprofloxacin concentrations were measured biologically by the cup plate agar diffusion method. Ciprofloxacin concentrations in serum and gynecological tissues were within the same range in both groups. Maximal serum concentrations of 6 and 4 mg/l, respectively, were recorded immediately after infusion. After 2 h serum concentrations ranged from 0.6 to 1.3 mg/l in both groups. At the same time, the tissue concentrations ranged from 0.62 to 3.3 mg/kg, indicating that tissue levels exceed corresponding serum concentrations. On average ciprofloxacin is concentrated in the extravascular space two-fold, as compared to the corresponding serum concentrations. There is no drug accumulation. The tissue concentrations obtained provide a full antibacterial coverage for gynecological infections, since the MIC for the most pathogenic bacteria is less than 1 mg/l.