Results of an exploratory study, using bone marrow suspensions, indicate a trend of accelerated recovery of bone marrow cells in guinea-pigs treated with heterologous spleen extract following exposure to a single, whole body dose of 650 r Co60 gamma radiation (LD79/20). A purified fraction of dog spleen extract which reduced radiation mortality, exhibited a similar trend. Erythrocytes, reticulocytes, granulocytes, megakaryo-cy tes and mitotic activity all appeared to be influenced in the same manner. The observations seem to support the contention of an induction of stem cell activity and possibly of accelerated maturation of various cellular elements of the bone marrow, as one of the mechanisms by which spleen extracts decrease radiation mortality.