RECENT measurements1–6 of the natural isotopic composition of potassium have yielded values for the potassium-39/potassium-41 ratio ranging from 13.48 ± 0.07 (Nier) to 13.96 ± 0.05 (Omura and Morito). These results are not inconsistent with the presence of variations of 1 or 2 per cent in the potassium-39/potassium-41 ratio for different samples. Earlier measurements, many of which have been summarized by Reutersward3, cover an even wider range, and include numerous reports7–12 of differences of 1 per cent or more between the potassium-39/potassium-41 ratios of minerals, normal and cancerous human tissues, and other biological samples. More recently, Voshage and Hintenberger13 have shown that potassium of abnormal isotopic composition can be produced naturally as a result of the action of cosmic rays on iron meteorites.