Supernatants obtained from short-cultured spleen cells (SCS) from BALB/c mice bearing a syngeneic mammary transplanted tumor--S13--showed enhancing activity on tumor growth when inoculated into the foot pad of normal syngeneic mice 24 hr before injection of S13 tumor cells. The present work was designed to characterize the spleen cell population responsible for the releasing of the enhancing factor (EF) as long as the tumor grows (small tumor bearing mice--STBM--and large tumor bearing mice--LTBM). Pretreatment of spleen cells with anti-Thy 1.2 serum + C' and nylon-wool columns were utilized to separate cell populations and to characterize the cellular source of the enhancing activity in the spleens of STBM and LTBM. In this tumor system, evidence is presented for distinct enhancing cell population operating in the spleens of STBM and LTBM. In early stages of tumor development, the EF was found to be associated with T and non-T cells, whereas in advanced stages of tumor growth, this activity was found to be associated with only T cells.