SUMMARYSerum raised against lymphocytes of one species has been shown to have a well marked effect on the ability of sensitized lymphocytes of a different species to respond to a specific antigen. Previous reports of species specificity may have resulted from relative insensitivity of the methods employed, as compared with the present “blocking” method. Species specificity of antilymphocyte serum (ALS) is probably considerably less than has been generally assumed.