We report on our experience with the application of recombinant human granulocyte colony-stimulating factor (G-CSF, Neupogen, Hoffmann La Roche) in patients with various types of hematological malignancies, who had chemotherapy-induced myelosuppression, and in patients with idiopathic aplastic anemia. The administration of G-CSF was associated with marked increase in white blood cells counts (WBC) in twelve out of 14 treated patients. In one patient with aplastic anemia the WBC decreased rapidly to the initial value after the cessation of cytokine therapy. Significant increase of platelet number was observed in 6 patients. No toxicity was encountered with the hematopoietic growth factor therapy. Our study points to the fact that G-CSF have a stimulating effect on the regeneration of hematopoiesis, particularly within the granulopoietic compartment. The effect can be obtained both in a case of idiopathic- and cytostatic-dependent marrow aplasia.