Corneal graft survival in rabbits was significantly (P less than 0 . 001) prolonged by topical treatment to the recipient eye with cyclosporin A 1% in arachis oil applied five times daily for 4 weeks. No graft was rejected whilst treatment was maintained but all grafts subsequently underwent rejection by the 64th postoperative day. All animals in a simultaneous control group in this fully masked study developed allograft reactions by the 23rd day. No local or systemic side-effects attributable to cyclosporin A were observed. It is believed that this is the first report of inhibition of an allograft reaction by cyclosporin A applied topically.