A phenotypically normal donor for bone marrow transplantation was found to have a previously unreported karyotype, 46, XY, t(18q+;22q-), resulting in a Ph'-like chromosome. Identification of the Ph'-like chromosome in cultures of skin fibroblasts, phytohemaglutinin-stimulated peripheral blood cells, and bone marrow cells from the marrow donor, but not in cell cultures from siblings or parents, indicated that it represented an acquired somatic mutation. Demonstration of the Ph'-like chromosome in the marrow graft recipient's blood and bone marrow cells after transplantation provided a unique and definitive marker of engraftment. Hematopoiesis appeared normal in both the donor and recipient after transplantation. This study indicates that a mutation creating a Ph'-like chromosome in hematopoietic cells need not produce hematologic abnormality. Presence of this translocation did not appear to interfere with normal hematopoietic or lymphoid differentiation and replication in the transplant setting.