Currently, the most commonly used grafts of progenitor and stem cells for patients undergoing bone marrow transplantation (BMT) are derived from large collections of autologous or allogeneic adult human bone marrow (BM). The feasibility of using human umbilical cord blood (HUCB), normal peripheral blood (PB), and smaller collections of BM as sources of hematopoietic stem cell grafts for adult patients remains questionable. We investigated the ex vivo proliferative potential of HUCB CD34+ cells as a means of expanding HUCB grafts, thereby making them more acceptable for clinical transplantation. HUCB-derived CD34+HLA-DR+ cells, maintained for 5 days in suspension cultures supplemented with 10% HUCB plasma and a combination of stem cell factor (SCF) and interleukin-3 (IL-3), displayed a 10-fold increase in the total number of CD34+ cells. In contrast, only a four-fold increase was observed in identical cultures initiated with BM-derived CD34+HLA-DR+ cells. Whereas BM CD34+ cells failed to proliferate in response to SCF alone, HUCB CD34+ cells expanded 5.6-fold by day 5, thus demonstrating an enhanced response to SCF. When the effects of SCF on the exit of HUCB cells from G0/G1 phases of cell cycle were investigated, we found that although HUCB CD34+HLA-DR+ cells were more quiescent than BM CD34+HLA-DR+ and BM CD34+HLA-DR- cells (97.5% of HUCB CD34+HLA-DR+ in G0/G1 vs. 88.6% of BM CD34+HLA-DR+ and 92.0% of BM CD34+HLA-DR- [p < 0.005]), HUCB CD34+HLA-DR+ cells exited from dormancy more rapidly than BM cells, such that by 36 to 48 hours following exposure to SCF, only 55% remained in G0/G1. Furthermore, an 8.4-fold increase in the number of HUCB CD34+ cells still residing in G0/G1 was observed on day 5 in cultures supplemented with SCF and IL-3, suggesting the generation of large numbers of primitive hematopoietic progenitor cells (HPC) in vitro. When the contribution of HUCB plasma to the exist of HUCB CD34+HLA-DR+ cells from G0/G1 phases of cell cycle was investigated, it was found that in serum-free media supplemented with only SCF or IL-3, HUCB cells did not exist G0/G1 as rapidly as when HUCB plasma or SCF plus IL-3 was present. In contrast, when HUCB plasma was added to any cytokine combination, it did not enhance the exist of BM CD34+HLA-DR+ cells from G0/G1 phases of cell cycle.(ABSTRACT TRUNCATED AT 400 WORDS)