90Sr was given at three different dose levels (29.6, 14.8 and 7.4 kBq/g b.w.) to groups of mice aged 300, 150, 75 and 25 days. It was found that the incidence of osteosarcomas was highest in the 75-day-old-group and lowest in the two oldest age groups. The frequency of lymphoreticular tumours was inversely dose-related (highest incidence in the lower dose series) and not dependent on age at the time of 90Sr injection. The frequencies of soft tissue tumours indicate that these tumours are more related to age than to the dose employed.