Thymectomy of Sprague-Dawley rats on the 3rd day of life failed to influence the time of onset, incidence, clinical, or histologic picture of runt disease produced by the intraperitoneal injection of adult Long-Evans spleen cells. The fact that severe immunologic impairment of the host by thymectomy does not modify runt disease was felt to be consistent with the current view that the if direction of the immunologic event in this syndrome is graft versus host.

Following the injection of 800 to 1000 million Long-Evans spleen cells into adult Sprague-Dawley rats, a severe illness comprised of dermatitis, gastrointestinal bleeding, arthritis, weight loss, and death ensued in 37 per cent of adults thymectomized neonatally and 13 per cent of normal controls. Histologic lesions were observed in 69 per cent of adequately thymectomized animals and 17 per cent of normal controls, and involved lymph nodes, spleen, liver, lungs, kidneys, joints, heart, and skin. The time of onset and the histologic and clinical pictures are consistent with the adult disease being a typical graft versus host reaction.

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