In a specially developed experimental model of an infected penetrating eye injury the importance of time and mode of antibiotic administration in the prophylaxis of post-traumatic eye infection was evaluated. It was concluded that the early administration of an antibiotic to which infecting organisms are sensitive can prevent the development of an inflammatory reaction in the majority of injured eyes. The late administration of the same antibiotic (6 h after injury) has no effect at all. Subconjunctival injection was found to be superior to intramuscular if the antibiotic was given at a similar time after injury. The results of this experiment stress the importance of very early antibiotic administration in the prophylaxis of an infection in penetrating eye injuries.