The restoration of immunological competence to irradiated adult mouse spleen was studied using an in vitro assay system. Competence of the spleen to evoke a graft versus host reaction in vitro was lost upon sublethal irradiation (550 R). Regeneration of immune competence occurred when such spleens were grown for 2–3 days in the presence of thymus tissue, but not when grown in the presence of a variety of other tissues. This thymic activity was demonstrated to occur across an intervening Millipore filter barrier. When lethal doses of irradiation were used, reactivation of immune competence did not occur unless both thymus and bone marrow were present. In this experimental situation the bone marrow appeared to supply the immunocompetent cells.

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