C3H mice (H-2 locus = k, hemoglobin D) received marrow cells intravenously following supralethal total-body x-irradiation. Donors were C3H, AKR (k, hemoglobin D), C57BR (k, hemoglobin S), A (a, hemoglobin D), and C57BL (b, hemoglobin S). Compatible marrow recipients suffered 20 % mortality. Incompatible H-2 and hemoglobin donation resulted in 97 % (mortality by the twenty-fifth day. H-2-compatible, hemoglobin-incompatible recipients suffered 67 % mortality by the fortieth day and then no further mortality. The H-2-incompatible, hemoglobin-compatible donation resulted in 36 % mortality by the fortieth day and showed continued mortality thereafter. Peripheral blood hematocrit and granulocyte counts and bone marrow cellularity and DNase activity revealed no significant differences, demonstrating successful grafts in all groups. However, the recovery of peripheral blood lymphocytes was inversely related to the mortality. Lymphopoiesis is apparently inhibited when the transplanted lymphocytes encounter overwhelming concentration of foreign antigens in the host. The H-2-compatible, hemoglobin-incompatible graft replaces the host's erythrocytes, thereby eliminating the inhibition of the graft's lymphocytic proliferation by that host antigen. During the first 40 days, homologous deaths result primarily from the effects of lymphopenia; thereafter, with recovery of lymphopoiesis, deaths are probably due to graft versus host reaction.