The CD34-positive population of bone marrow cells contain not only committed myeloid, erythroid, megakaryocytic and lymphoid progenitor cells but also more primitive stem cells capable of initiating haemopoiesis in long-term marrow culture that are termed long-term culture initiating cells (LTC-IC). Recently, different subpopulations of BM CD34+ cells have been defined. Thus the CD34+/CD45RO+ fraction contains both BFU-E and LTC-IC whereas CFU-GM are mainly CD34+/CD45RA+ and CD34+/CD33+. As human umbilical cord blood (UBC) also contains CD34+ cells and has been used as a source of material for clinical transplantation, we have undertaken a comparative study of the expression of CD34RO, CD45RA, CD33 and CD38 on CD34+ cells from UCB and BM using directly conjugated antibodies and flow cytometry. The frequency of CD34 cells in UCB (mean 1.0% +/- 0.3%) was similar to that in harvested pelvic BM (0.8% +/- 0.4%). The frequencies of CD34+ cells co-expressing CD33 (UBC: 54.1% +/- 28.7%; BM: 52.9% +/- 18.5%) and CD45RA (UBC: 24.8% +/- 16.6%; BM: 21.1% +/- 19.7%) were not significantly different between UCB and BM. In contrast, a significantly higher percentage of UCB-CD34+ cells co-expressed CD45RO+ (19.7% +/- 10.3%) compared with BM-CD34+ cells (7.2% +/- 5.3%; p < 0.001). The absolute number of CD34+ cells in UCB correlated with the number of committed myeloid (CFU-GM) and erythroid (BFUe) progenitors; a similar relationship was found for the absolute number of CD34+/CD33+ cells. However, the numbers of CD34+/CD45RO+ cells showed no correlation with numbers of committed progenitors.(ABSTRACT TRUNCATED AT 250 WORDS)