A 45-year-old woman with severe idiopathic marrow failure was prepared for marrow transplantation by administration of cyclophosphamide (cy) 50 mg/kg on each of four successive days. She then received an intravenous infusion of 20 X 10(9) nucleated marrow cells from an HL-A matched and mixed lymphocyte culture (MLC) non-reactive sister. There was evidence for minimal marrow recovery in 1-2 months and a second marrow infusion was carried out 69 days after the first without additional immunosuppression. There was a continued slow recovery of peripheral blood counts with complete reconstitution of erythropoiesis, return of the white blood cell count to between 3 and 4000/mm3, with 50% granulocytes, and platelets to 60--70,000/mm3, 11 months after the initial grafting attempt. Red cell antigens and gamma globulin allotypes were of recipient type. The MLC and the indirect cell mediated lympholysis (CML) test became positive possibly indicating cellular sensitization to non HL-A antigens. This report of a patient with severe marrow failure documents autologous recovery of marrow function after receiving a large dose of cy and allogeneic marrow. The implications of this are discussed.