SUMMARYStrain A mice were rendered tolerant to (A × C57BL/1)F1hybrid tissue by means of a heavy dose of total body X-irradiation and injection of hybrid spleen cells, in order to produce a chimera. These chimeras were then injected with spleen cells from strain A mice; this resulted in a runting syndrome which was characterized by weight loss, hematological depression, and depletion of lymphocytes in the lymphoid tissue. The hematological and pathological changes observed in these animals were similar to those in (A × C57BL/1)F1hybrid mice undergoing runting as a result of the injection of strain A spleen cells, except that the dermatitis and intestinal lesions regularly observed in classical runt disease did not occur in the wasting disease produced in syngenic mice.