In from nine to 14 days, rabbits previously sensitized to their donors' tissues started to reject the rabbit corneal buttons that had been placed in their vascularized corneas. Systemically administered antilymphocyte serum (ALS) significantly retarded or prevented the development of the graft reaction during a 40-day observation period. Animals receiving normal horse serum or ALS systemically developed signs of anaphylaxis and serum sickness after repeated injections.