By the use of germfree animals we plan to continue the determination of the effect of microorganisms on the response of mammals to wholebody ionizing radiation. We are attempting to obviate the histopathology of graft-vs-host disease observed in germfree radiation bone marrow chimeras by pre-treatment of the bone marrow inoculum with anti theta serum. We are evaluating the immunological competence of these pre-treated allogeneic chimeras, and we are attempting to define their apparent T cell defect in terms of an excess of suppressor cell activity. We are also attempting to develop animal models for immune deficiency disease by studying the athymic mouse, the asplenic mouse, and a hybrid of the two mice in the germfree state. When these immune deficient models are developed, we will try appropriate reconstitution procedures, e.g., fetal thymus, fetal liver, or bone marrow transplantation. Bibliographic references: P.M. Bealmear, E.R. Richie, and J.J. Trentin. 1974. Histological survey of radiation chimeras repopulated with spleen and bone marrow cells pre-treated with Fab fragments of antithymocyte (ATG). Exp. Hematol. 1: 293-294; D.A. Freedman, J.R. Montgomery, R. Wilson, P.M. Bealmear, and M.A. South. 1975. Further observations on the effect of reverse isolation from birth on cognitive and affective development. J. Amer. Acad. Child Psychiatry.