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Low zone tolerance (LZT) of mice to bacteriophage fd has been further investigated and found to be specific for the antigen and inducible within 24 h. LZT and immunity are retained when spleen cells from tolerant or immune animals are transferred into thymectomized, lethally irradiated recipients reconstituted with fetal liver. Thymocytes, educated in vivo with antigen doses inducing LZT, are capable of transferring this tolerance into normal syngeneic mice and maintaining it there specifically. Treatment with anti-[Theta] serum abolishes the suppressive capacity of the educated thymocytes. The data suggest that LZT is actively maintained by living T cells.