In the present work, we tested the hypothesis that liquid cultures (LCs) ofcord blood CD34+ cells at an appropriate low O2 concentration (3%) couldsimultaneously allow colony forming cell (CFC) expansion and long termculture initiating cell (LTCIC) preservation.We found that 3% was the minimal O2 concentration, still allowing thesame rate of CFC expansion as at 20%. We report here that 7 daysLCs(liquid culture) of cord blood CD34+cells at 3% O2 maintain LTCICbetter than 20% O2 and allow a similar amplification of CFCs (20-55 folds)without modifying the CD34+ cell proliferation.The fold expansion of both the CD34+ and the CD38+ cell population washigher in 20% O2 concentration, this denotes the expansion in 20% O2concentration involved a certain degree of maturation without preservationof the early subset (CD34+, CD38-).These results suggest that low O2 concentration similar to those found inbone marrow participates in the regulation of haematopoiesis by favouringstem cell-renewing divisions. This expansion method that avoids stem cellexhaustion could be of paramount interest in hematopoietic transplantationallowing the use of small sized graft in adults.