Late mortality and pathology were assessed in various mouse strains following total-body irradiation (TBI) and bone marrow transplantation. A, C57BL/6, B6AF1, LP and C3H mice received TBI in two fractions 3 h apart at total doses of between 11 and 15 Gy. They were then transplanted with syngeneic bone marrow cells providing sufficient reconstitution to avoid hemopoietic failure. Long-term survival data revealed both radiation dose- and strain-dependent onset of mortality between 1 and 2 years post-treatment. Renal damage appeared to have contributed to the late mortality in most treatment groups as shown by glomerular lesions, elevated blood urea nitrogen and an accompanying fall in hematocrit. Hemolysis was deduced to be the major cause of anemia, as concluded from results of 51Cr-labeled erythrocyte survival. No decrease in erythropoiesis was evident as seen from spleen and bone marrow 59Fe uptake. These findings are together consistent with the manifestation of a hemolytic uremic syndrome (HUS) with kidney glomeruli representing the principal sites of injury responsible for both renal dysfunction and microangiopathic hemolysis.