Twenty-two patients undergoing penetrating keratoplasty received either topical fortified gentamicin (13.6 mg/mL) or a loading dose of intramuscular gentamicin before surgery. Cornea and aqueous humor samples were obtained and assayed by radioimmunoassay for gentamicin. In the topical group, the average gentamicin levels in the cornea and aqueous were 16.2 micrograms/g and 0.3 microgram/mL, respectively. Parenteral drug levels were less variable and averaged 6.1 micrograms/g in the cornea and 0.4 microgram/mL in the aqueous. Although this study demonstrates that parenteral therapy can be used to deliver adequate drug levels to the cornea, it requires far more antibiotic and subjects the patient to potentially toxic systemic side effects. Therefore, topical administration is the preferred route of delivery of antibiotic to the cornea.