Effects of water with varying isotopic composition (high and low deuterium content) were studied on the model of cultivated human endothelial cells. In laboratory conditions, changes in the isotopic content of water in nutrient substrate did not affect significantly the cell proliferative activity. In cells cultivated in no-serum medium increased deuterium concentration inhibited growth with a consequent of cell death. Both decreased and increased concentration of deuterium activated proliferation in originally low-density cell cultures supplemented with serum. It is hypothesized that changed isotopic composition of water has an effect only when combined with some other cell cultivation factors.