Hemopoietic CD34+ progenitors were isolated by immunomagnetic method from normal bone marrow (BM) or from peripheral blood (PB) of patients with non-Hodgkin's lymphoma treated with chemotherapy and granulocyte colony-stimulating factor (GCSF). Aliquots were seeded in long-term cultures (LTC) on bone marrow-derived stromal layers; non-adherent and adherent clonogenic content of the cultures was assayed weekly. The final recovery and the clonogenic efficiency of the CD34+ cells were slightly higher in PB samples than in BM controls. In long term cultures PB cells sustained hemopoiesis as much as BM cells; at week 3 and 4 PB total mononuclear cells and CD34+ cells showed a non-adherent cell recovery higher than the respective BM controls. Furthermore, PB CD34+ cells were expanded in liquid culture in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) or G-CSF alone or combined with interleukin 3 (IL3), stem cell factor (SCF), interleukin 1 (IL1), interleukin 6 (IL6). The combination of GM-CSF, IL3, SCF, IL1 and IL6 produced the maximum increase of both mononuclear cells (30-fold) and granulocyte-macrophage colony forming units (CFU-GM) (4.6-fold) after 7 days of cultures; yet after 14 days a strong decrease of the CFU-GM occurred. These data suggest that G-CSF following chemotherapy mobilizes both early and committed hemopoietic progenitors.