Spleens of mice which had been treated with E. coli endotoxin or typhoid-paratyphoid vaccine were extracted with isotonic saline. The extract was filtered through an asbestos filter and chromatographed on a Sephadex G-200 column. The fraction which was excluded at around 2.5-void volume (molecular weight, about 20,000) was significantly effective in increasing survival of animals when it was repeatedly injected in mice after x irradiation (600 R). The injection caused an increase of the weight of spleen of the animals with an increased number of endogenous spleen colonies. The result suggests that the life-saving effect of the spleen extract is based on its effect of stimulating repopulation of autochthonous hematopoietic cells in the x-irradiated animals. A similar splenic extract prepared from normal mice had an ambiguous effect on the survival of x-irradiated animals.