OBJECTIVEThe purpose of the present study was to investigate the relation between the dose of tumor cell inoculation (especially the doses less than minimum required to evoke tumor growth) and the anti-tumor immune system, particularly lymphoblast formation and cytotoxic activity of lymphcytes.

METHODWe inoculated rats with various doses of SST-2 tumor cells and examined natural killer (NK) cell activity and lymphoblast formationin vitro.

RESULTThe results showed that the cytotoxicities against SST-2 cells and lymphoblast formation of lymphocytes were enhanced by small dose inoculation of tumor cells that could not induce tumor growth.

CONCLUSIONIt was suggested that was lymphocutes play an important role as an anti-tumor immune system at small doses of tumor inoculation, which appears to reflect an early stage of tumor growthin vivo. It was also suggested that SST-2 tumor inoculation might be a useful model for studying the anti-tumor immune response in SHR rats.