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The antigen inducing antibody formation after the inoculation into the organism can, under suitable conditions, elicit quite the contrary reaction — specific immunological unresponsiveness or immunological tolerance. The critical factor deciding upon which of the reaction pathways will take place is the amount of antigen. Small doses lead to immunity, whereas large doses of appropriate antigen can induce tolerance. The second type of the immunological reaction to the introduction of antigen can be most easily shown in such an experimental model where the immunized animal is immunologically immature and the material used as antigen is not too foreign to the recipient. The third factor enhancing inducibility of tolerance is the use of antigen which is represented by cells capable of further multiplication in the recipient. Such conditions can be fulfilled if the isoantigen is used, i. e. the cells from another individual of the same zoological species, and the embryo or the newborn animal serves as recipient.