Continuous extracorporeal irradiation of the blood (ECI) for 10 to 21 days was carried out in baboons by means of an x-ray irradiator without a pump or anticoagulation. One baboon received a daily cumulative dose of 20,000 R to the red blood cells and died on the tenth day with severe hemolysis. Baboons given 10,000 R daily tolerated the irradiation well but showed severe hemolytic anemia. Prompt, pronounced, and long-lasting lymphocytopenia was observed. No consistent changes in platelet or granulocyte levels were noted. By the tenth day lymph nodes in all baboons had undergone atrophy involving both germinal centers and adjacent areas. Lymphoid hypoplasia lasted for more than 2 months. Allogeneic skin grafts survived for 7 to 12 days in untreated baboons. In a treated baboon, allogeneic skin grafted on day 2 of ECI was rejected after 15 days despite continuous ECI. Three treated baboons received skin grafts on day 10 and showed graft survival of 41 to 53 days despite discontinuation of ECI. No difference in antibody formation against sheep red blood cells was found between ECI-treated and untreated baboons. Surviving animals have been observed for more than 300 days without apparent late sequelae.

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