Antiviral agents are, in many ways, similar to antibiotics. They inhibit virus multiplication and can make the use of corticosteroids safer by preventing the reactivation of latent virus and controlling the tendency of virus to multiply in the presence of corticosteroids. Like antibiotics, resistance to these agents can develop and the agents may have some effect on the host. Cytosine arabinoside (CA) is a therapeutically active antiviral agent which has the ability to alter the course of viral and cell multiplication.1 In addition to being able to cure experimental herpes simplex keratitis, CA has been extremely useful in treating patients with herpetic keratitis which is resistant to therapy by idoxuridine (5-iodo-2′-deoxyuridine, IDU).2 Although CA is more soluble than idoxuridine and allows preparations to be used less frequently than idoxuridine, it has the disadvantage of producing, under some circumstances, a reversible corneal toxicity. Corneal toxicity was first observed in