Rabbits with inactive retinochoroiditic scars due to experimental toxoplasmosis were subjected to a series of intravenous inoculations of antilymphocyte serum. Two out of five rabbits so treated developed reactivation of their retinochoroiditic lesions. All of a similar group of rabbits treated with intravenous inoculations of normal horse serum developed reactivations of their lesions. Some mechanisms considered responsible for the breakdown of these healed lesions are discussed.