A mouse lymphoma cell line has been established that is dependent for growth on the presence of an activated serum component. This growth factor is found in an inactive form in fetal bovine serum (FBS) and can be activated by 2-mercaptoethanol and by macrophages. A factor-containing fraction can be separated from FBS by Sephadex G-100 chromatography and once activated, can be used as a substitute for whole serum in the culture medium without adverse effect on the cell growth. The significance of these findings with respect to the study of the immune response and cancer research is discussed.