Hematopoietic stem cells, CD34 positive cells, were isolated from the peripheral blood of three patients with malignant lymphoma, and were cultivated in suspension for 14 days in the presence of cytokines, including granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, interleukin-3 and stem cell factor. The stimulation of cell proliferation and differentiation into mature neutrophils was most effective when all these cytokines were used in combination. Mature neutrophils differentiated in vitro gained the ability to ingest latex spheres and to produce hydrogen peroxide in response to phorbol myristate acetate. These findings raise the possibility that the prolonged neutropenia following high dose chemotherapy could be ameliorated by the transfusion of autologous neutrophils expanded and differentiated in vitro.