IN Science of February 16, 1934, Prof. Gilbert N. Lewis summarises the results of certain sporadic attempts to observe the effect of water containing heavy hydrogen, H2, upon living organisms. Experiments have necessarily been confined to small organisms, though some preliminary observations on mice are included. The first experiments were upon tobacco seeds, the germination of which was completely retarded by pure HO and slowed up some 50 per cent by water containing 50 per cent HfO. Seeds transferred to normal water after three weeks in pure HgO sprouted in about half the cases but gave unhealthy seedlings. Yeast cultures in an appropriate nutrient medium dissolved in pure heavy water failed to grow, and Pacsu has also shown that the evolution of carbon dioxide by yeast from sugar solution made up with heavy water is much diminished.