The effect of age on the sensitivity of man to ionizing radiations has for many years been recognised by radiotherapists who, on a somewhat empirical basis, have planned their treatment to take this factor into account. In animal work the importance of age as a factor influencing radiation sensitivity had virtually been neglected until a few years ago. One still finds papers describing work with animals, in which the age changes are relevant to the radiation effect studied, and yet the age of the animal is not even mentioned. Such neglect can lead to misleading interpretations, particularly when the effect studied is not large. There are two difficulties in attempting to take the age factor into account in the experimental design: (a) our knowledge of the normal variation with age of form, function and composition is inadequate; (b) the obtaining of large numbers of animals of a specified age is costly and time consuming. However, in proportion to the cost of most biological experiments, this additional b...