There are two particular questions about radiation-induced mutations in mammals which should be of interest to radiologists: (1) How likely are mutations to occur, as a function of dose? (2) How serious are their effects? Let us first remind ourselves of the main classes of mutations recognized by geneticists. Figure 1 shows a hypothetical pedigree involving a dominant gene mutation; Figure 2 represents a recessive gene mutation, and Figure 3 an inherited semi-sterility caused by a chromosome translocation. How frequently such changes occur in mammals as a result of exposure to radiation is still far from accurately known. The published data, all obtained from mice (Table I) indicate that, for single large exposures, inherited semi-sterilities are the main genetic effect to be expected, occurring in a small percentage of sperm for each 100 r. The gene mutation rates are not very reliable, since they are calculated from a few mutations in a small number of tested cells (Table II). For practical application...