IFNα alone or in combination with retinoids or haematopoietic growth factors has been used to treat patients with early MDS because of its properties as a differentiation inducing agent. We investigated whether treatment of patients with refractory anemia (RA) with IFNα (1.5×106 IU twice a week) and intermittent all-trans retinoic acid (ATRA, 25 mg/m2/d) influences in-vitro megakaryocytic (MK) proliferation and differentiation stimulated by PEG-rHuMGDF. Low-density non-adherent bone marrow (BM) cells from 8 patients with RA were assayed prior to any treatment other than supportive and after a period of 6 months of treatment. MK development was assayed in suspension cultures in the presence of PEG-rHuMGDF and SCF for 7 d using morphological criteria and flowcytometric analysis of CD42b (GP1b) positive cells. BM-cells from 10 healthy individuals served as control. Following stimulation with PEG-rHuMGDF 23±7% and 16±4% of control cells were CD42b positive after 5 and 7 d of cultures, respectively. In cultures of cells from MDS patients prior to treatment 8±2% and 7±3% of cells were CD42b+ on days 5 and 7. In the course of IFNα treatment cultures of all BM samples from these MDS patients revealed a significant reduction of MK precursor cells (3±2% CD42b+, p=0.03 and 0.04). In conclusion, treatment with IFNα and ATRA did not result in improved megakaryocytopoiesis as assessed by in-vitro cultures. On the contrary, low-dose IFNα appears to suppress cell proliferation as well as MK development.