A stable, non-radioactive isotopic form of water, containing 2 atoms of deuterium (D) and one atom of oxygen (2D2O), with DNA-labeling activity. Upon ingestion of deuterium oxide, 2H is incorporated into the deoxyribose moiety of DNA of newly divided cells. Rapidly dividing cells, as in the case of B-cell chronic lymphocytic leukemia (B-CLL), can be labeled with deuterium oxide and measured using gas chromatography and/or mass spectrometry.