SUMMARY Groups of male CBA-mice were treated with oestrogenic hormones alone or in combination with 85Sr and 90Sr. The strontium retention was measured in special skeleton parts and also by means of whole body counting. After a period of increasing bone weight in mice treated with oestrogen and 90Sr, there were signs of a serious breakdown of that newly formed "oestrogenic" bone tissue. The strontium retention was in no case found higher in animals treated with both the hormones and 90Sr when compared to those treated with 90Sr alone. The life span was shortened in mice given the combined treatment. The results sort the theories that at the same time irradiated and stimulated cell populations are exposed to a larger risk for induction of tumours and that the irradiation injury cannot be referred entirely to the irradiation dose in a physical sense.