We investigated the origin of the fibroblastic compartment of stromal hematopoietic microenvironment in eight chronic myeloid leukemia (CML) patients following allogeneic BMT. At the time of the study, all eight CML patients showed complete and long-lasting (14-87 months) engraftment of donor hematopoiesis and absence of clonal Ph-positive hematopoiesis. The study was carried out using in vitro amplification of informative DNA sequences: a Y chromosome specific DNA fragment in three patients who received a sex-mismatched allograft, and locus D1S80, a variable number of tandem repeats polymorphism, in five patients who received a sex-matched allograft. In all cases bone marrow fibroblasts were of recipient origin. These data indicate that with current BMT procedures the stromal compartment of hematopoiesis is not transplantable in humans.