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Irradiated rats were reconstituted with bone marrow from F1 hybrids. Ia antigen of donor-bone marrow origin was detected by an immunoperoxidase technique on cryostat sections and found predominantly in the medulla of rat thymus 2 wk after reconstitution. These Ia-bearing cells increased in number with time after reconstitution, but the Ia on the cortical epithelial cells remained of host origin. The nature of the bone marrow-derived cells and their implication for major histocompatibility complex restriction are discussed.
Barclay et al. (Mon,) studied this question.