Your correspondents Howes and Field (1968) have shown that some recent results on the effects of fractionated doses on experimental solid tumours agree with the data of Friedman and Pearlman (1955) on the treatment of chest nodules in humans. Howes and Field consider that the differences between fractionated dose effects on skin and on these solid tumours can be largely explained on the basis that the solid tumours contain a percentage of hypoxic cells that become re-oxygenated following the first radiation fraction.