Nineteen patients about to undergo elective vitreous surgery received 1 oral dose of 750 mg of ciprofloxacin before surgery. Specimens of serum and vitreous were collected 90 minutes to 18 hours after drug administration and were assayed for antibiotic content with a microbiologic disk agar technique. From 4 hours and 50 minutes to 16 hours and 50 minutes after a single oral dose, ciprofloxacin reached intravitreal levels above its minimal inhibitory concentration for 90% of Staphylococcus epidermidis, Bacillus species, and Enterobacteriaceae. However, intravitreal levels never exceeded the MIC90 for Staphylococcus aureus and Pseudomonas.