We investigated the origin of bone marrow fibroblasts in three bone marrow transplant recipients with aplastic anaemia and leukaemia who received grafts from HLA-identical siblings of opposite sex. The patients were conditioned for transplantation with high doses of cytotoxic drugs and 300--1000 rads total body irradiation. After transplantation, bone marrow cells were cultured in T flasks for 3 weeks and the adherent cells were then trypsinized and passaged weekly. After several passages the cells had the typical morphology and growth pattern of fibroblasts. Metaphases from these cells were all of recipient sex type. In contrast, haematopoietic cells and lymphocytes obtained at the same time were of donor sex type. Our findings indicate that the human bone marrow fibroblast is not derived from a precursor common to haemotopoietic cells or lymphocytes. The bone marrow fibroblast appears to be a mesenchymal cell, unrelated to haematopoietic stem cells, which is capable of in vitro proliferation after as much as 1000 rads of total body irradiation.