Treatment with a combination of chemotherapy and G-CSF leads to the release of hematopoietic stem cells from the bone marrow (BM) to the peripheral blood (PB), where they can be harvested for transplantation. Premobilization BM CD34+ cells were reported to proliferate actively, while virtually none of the mobilized PB CD34+ cells were in the S/G2M phase. We were interested in elucidating the cell cycle state further and in investigating the role of adhesion molecule expression on marrow-adherent and circulating CD34+ cells during different phases of the cell cycle. Consecutive premobilization BM and leukapheresis product (LP) samples were obtained from 14 patients following G-CSF-supported chemotherapy. Steady-state BM and LP CD34+ selected cells were triple-stained for CD34, for DNA using the intercalating dye 7-aminoactinomycin D, and for Ki-67, cyclins, or adhesion antigens. Ki-67 is expressed in all phases of the cell cycle except G0 and was found in 69.14%+/-3.46% (mean +/- standard error [SE]) of BM CD34+ cells and 62.78%+/-3.37% of LP CD34+ cells, while in BM significantly more CD34+/Ki-67+ cells were in the S/G2M phase of the cell cycle than in LP (8.6%+/-0.9% versus 1.8%+/-0.3%, respectively, p = 0.0001). Therefore, most circulating mobilized CD34+ cells are in the G1 phase, similar to their steady-state BM counterparts. Cyclin A became detectable in the 2n DNA peak. As expected, a higher proportion of CD34+/cyclin A+/S/G2M cells was found in BM than in LP (p < 0.05). Antigen density of the cyclins D3 and D2 tended to be higher on LP than on BM CD34+ cells, while D1 was found at low levels in similar density. The adhesion antigens CD18, CD49b, CD49d, CD49e, CD58, and CD62L were expressed in a significantly higher proportion of S/G2M-phase than in G0/G1-phase CD34+ cells. The strongest association to the proliferative status was observed for CD49d, which was coexpressed by 85.9% +/-2.6% (BM) or 90.8%+/-2.5% (LP) of CD34+/S/G2M cells, whereas a distinct CD34+/CD49d-/S/G2M population could not be detected. The average coexpression of the other antigens was 57% (CD49e, CD18) or lower. Our results demonstrate that the majority of PB CD34+ cells mobilized following G-CSF-supported chemotherapy and steady-state BM CD34+ cells are in the late G1 phase of the cell cycle and show a correlation between the expression of adhesion receptors and cell cycle status of CD34+ cells in both BM and LP.