During bone marrow engraftment following BMT there is a re-establishment of fetal erythropoiesis, expressed by the increase of F-cells. This seems to depend on several factors such as underlying disease, conditioning before therapy and other mechanisms concerning both the donor and the recipient bone marrow. The aim of this work was to study the factors influencing F-cell production during bone marrow engraftment following transplantation. We studied 28 patients who underwent allogeneic bone marrow transplantation, for various hematological malignancies (CML, AML, ALL, CMML and SAA). F-cells were estimated on peripheral blood smears by indirect immunofluorescence. Overall, there was an F-cell increase after BMT in comparison with values before BMT; this increase was significant on days 15-50 (p <.01). F-cell on days 18, 25, 32 and 40 following transplantation were significantly higher (p <.01) in patients who have had increased F-cell numbers post-chemotherapy before BMT, compared with the patients who did not show any increase of the F-cell number post chemotherapy. During the first month following transplantation (day 7 to day 40) patients who were transplanted from high F-cell donors failed to show any significant differences in their F-cell numbers in comparison to those transplanted from low F-cell donors. However, the F-cell increase became significantly higher in the former group between days 50 and 120. This observation implies that the stressed erythropoiesis of the initial phase does not allow revealing the varying F-cell production of the capacities donor bone marrow, while later, when the graft has settled, the high F-cell donors reveal this property of the host.