Infection of Sr90-treated mice by BCG at a time close to the expected appearance of the first bone tumors resulted in a delay of the development and a significant decrease of the total incidence of such tumors. Neither the osteogenic or osteolytic character of the tumors, nor their distribution in the skeleton, nor the survival time of the animals was affected by BCG. On the other hand, an increase of leukemia incidence could be noted in the infected animals.