[en] In the presence of tritiated water (HTO), Escherichia coli NG30 was grown in sealed test tubes containing synthetic medium (M9 - 0.4 % glucose-0.005 % arginine), and cell growth was monitored by measuring medium turbidity. Catalase (60 μg/ml) was added to the medium to reduce HTO associated accumulation of H₂O₂. A positive correlation between increasing concentration of HTO in the medium and the growth suppression was seen. A similar trend was observed when cells were grown under continuous irradiation of ⁶⁰Co gamma-rays. The RBE of HTO beta-rays relative to gamma-rays was calculated from the accumulated doses to the cultures. It was found to be 1.7 when the growth was inhibited to 60 % of the control, and increased with decreasing dose. (author)