Immunopotentiated rats, which were injected with Propionibacterium acnes or BCG, had the 50% survival twice as long as those in untreated controls after intravenous inoculation of Sato lung carcinoma (SLC) cells. The amount of labeled tumor cells in the lung of the adjuvant-treated rats decreased significantly in the first 20 hr after intravenous injection of 51Cr-labeled tumor cells compared to that of control animals. The elevated activities of ATPase and acid phosphatase in the whole nucleated spleen cells as well as spleen lymphocytes separated by Ficoll-Conray gradient were also demonstrated in adjuvant-treated groups. These data suggested that the elevation of ATPase and acid phosphatase activities in nucleated spleen cells as well as spleen lymphocytes has an important role for the suppression of tumor growth in adjuvant-treated rats.