Eight hundred and twenty male CBA-mice, 75 +/- 3 days of age were divided into four main series. Three of these; A, B and C were further divided into three subgroups. To these 90Sr was given at three different dose levels alone (A) or in combination with either oestrogen (B) or prednisolone (C). In one series (D) all animals were given only oestrogen. The development of intramedullary oestrogen induced bone formation and the early events of bone tumour induction are reported. The 90Sr activities were selected in such a way that the lowest one would be below, the intermediate beyond and the highest well above the limit of bone tumour induction. Comparing the 90Sr injected animals with those given also oestrogen revealed that oestrogen had no promoting effect at the lowest and intermediate dose levels but increased the tumour incidence with a factor 4 at the highest dose level whereas prednisolone had an inhibitory effect. From the results obtained it was proposed that oestrogen may act only as a preparatory promoter and not assist in the initiating events. As has been reported elsewhere, the frequency of osteoblastic and osteoclastic osteosarcomas was higher in animals given oestrogen and 90Sr than 90Sr only. Mice given only oestrogen did not develop bone tumours.