Cell proliferation of four leukemia cell lines with no treatment under normoxia or hypoxia. 2 × 105/ml cells in 0.5 ml RPMI1640 + 3% FCS were cultured under normoxia (21% O2) or hypoxia (1% O2) for 48 hours in triplicate. Then, cell count was measured after 24 and 48 hours, respectively. NB4 (A) significantly diminished their proliferation under hypoxia compared with normoxia (48 hours) (p = 0.0001). In contrast, THP-1 (B) significantly increased more under hypoxia than normoxia (48 hours) (p = 0.005). Kasumi-1 (C) and HL-60 (D) did not show significant difference of cell growth between normoxia and hypoxia (48 hours). ▲: under hypoxia, ●: under normoxia. *** p < 0.001, ** p < 0.01, ns p > 0.05 (unpaired, two-tailed t-test).