Thirty five patients (mean age: 70 years) undergoing cataract surgery or vitrectomy were perfused with ciprofloxacin (400 mg IV/for 1 hour) or received either a single, 2, 6 or 8 preoperative orale doses (750 mg each). The samples of serum, aqueous humor or vitreous fluid were taken simultaneously at 1, 2, 3, 4, 6, 9 or 12 hours after the last administration of antibiotic. The antibiotic concentrations were determined by microbiological techniques (using Bacillus subtilis ATCC 6633). Aqueous humor levels, after perfusion of 400 mg, were stable during 3 hours (mean: 0.22 mg/l); after a single oral dose of 750 mg were stable during 12 hours (mean: 0.18 mg/l) but were significantly lower than after multiple doses (up to 0.8 mg/l) which suggest accumulation in anterior chamber. In vitreous fluid, levels were elevated from one to six hours after the end of the perfusion (up to 0.4 mg/l). The levels are above the MIC 90 of enterobacteriaceae, and around the MIC 50 of Staphylococcus or Pseudomonas.