A therapeutic attempt at immunologic reconstitution of an infant with thymic dysplasia ("Swiss agammaglobulinemia") by means of a maternal bone marrow transfusion is recorded. The result, a fatal graft versus host (GVH) reaction, was characterized (1) clinically by edema, erythema, splenomegaly, and hepatomegaly, (2) in laboratory tests by acute, severe anemia, thrombocytopenia, and a decrease in serum protein, and (3) pathologically, by edema, histiocytic infiltration, and erythrophagocytosis. Therapeutic possibilities in such patients are discussed.