From a four-way cross of inbred strains of mice, two separate selection experiments were performed: two-way selection for survival under 100 r/day of x rays and twoway selection for survival under 400 r/day of x rays. Selection was successful in both experiments but was much more pronounced under 100 r/day. A possible inverse relationship of intensity of stress and heritability is suggested. Body weight had a low positive phenotypic correlation and a high genetic correlation with survival under 100 r/day. Litter size was also a correlated response in both selection experiments. We conclude that body weight is not important per se in conferring greater resistance to irradiation but is rather a component of fitness in this genetically heterogeneous stock. Body weight is likely genetically correlated with radiation resistance because they are both components of fitness. It was not possible to establish whether an asymmetrical response in the selected trait had occurred, but it was apparent that the correlated response in body weight was in the low direction only. Possibly two-way selection has brought about a difference between the lines mainly through selection for a general lowered fitness in the low line alone. Females were found to be more resistant thanmore » males under 400 r/day, but the reverse was true under 100 r/day. (auth)« less