PURPOSE: The aim of this study was to find the dose of agonistic 4-1BB monoclonal antibody (mAb) that results in optimal T cell activation. METHODS: Cancer was induced in mice by an intrahepatic parenchymal injection of 1x10(5) cells of CT26 cells. Cancer-carrying mice (n=84) were divided into seven groups and treated with either rat IgG or agonistic 4-1BB monoclonal antibody (mAb) (5microgram, 10microgram, 20microgram, 100microgram, 200microgram, or 300microgram). All treatments were administered intraperitoneally on days 7, 9, and 11. Mice from each group were sacrificed on days 14, 28, and 42. Harvested livers were weighed and the numbers of T cells in the splenocytes were analyzed with a FACS Vantage flow cytometer. RESULTS: Liver weights increased when 5microgram of agonistic 4-1BB mAb was administered, but showed no additional weight increase for doses greater than 10microgram. The absolute numbers of CD4+ and CD8+ T cells increased in groups treated with low doses of agonistic 4-1BB mAb (5microgram, 10microgram, or 20microgram), but did not increase in the groups treated with high doses of mAb (100microgram, 200microgram, or 300microgram). The levels of CD4/annexin V and CD8/annexin V increased as the dose increased, and the absolute cell numbers of CD4/annexin V were greater than those of CD8/annexin V. CONCLUSION: Liver weight, including the cancer mass, failed to increase at agonistic 4-1BB mAb doses greater than 10microgram. A high dose (> or =100microgram) of agonistic 4-1BB mAb resulted in lower counts of absolute T cells. This study suggests that a low dose (20microgram) of agonistic 4-1BB mAb can be used for optimal T cell activation in combination with other anti-cancer treatments.