Rats thymectomized and irradiated as adults were restored to immunologic reactivity by grafts of normal adult rat thymus and bone marrow. Reactivity of the delayed (cellular) type and formation of mercaptoethanol-sensitive (MES) and mercaptoethanol-resistant (MER) antibody returned within 3 weeks, while Arthus reactivity remained subnormal till 9 weeks after irradiation and grafting. When the thymus donor was tolerant to BGG, the recipient showed specific non-reactivity to this antigen 3 weeks and, to a much lesser extent, 6 weeks after grafting. This non-reactivity affected delayed responses and MER antibody. No effect was noted on Arthus reactivity and a slight effect on MES antibody. Controls showed that the non-reactivity was not due to transfer of free antigen at the time of grafting. It was concluded that different source organs are responsible for different immune functions and that specific immunologic tolerance may be induced within such an organ as the thymus.

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