Thujone, a naturally occurring monoterpene, was found to enhance the total WBC count, bone marrow cellularity, number of α-esterase positive cells, number of plaque forming cells in spleen and circulating antibody titer in Balb/c mice (1 mg/kg body weight, intraperitoneally for 5 days). Thujone treatment enhanced proliferation of splenocytes and thymocytes, both in the presence and absence of specific mitogens. Administration of Thujone was found to stimulate the cell-mediated immunological response in normal and tumor bearing Balb/c mice. A significant enhancement in natural killer (NK) cell mediated cytotoxicity, antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent complement mediated cytotoxicity (ACC) in both normal as well as tumor-bearing animals was observed after the administration of Thujone. Production of cytokines such as IL-2 and IFN-γ was significantly enhanced by the administration of Thujone. The stimulatory effect of Thujone on cytotoxic T lymphocyte (CTL) generation was determined by Winn's neutralization assay using CTL sensitive EL4 thymoma cells. Thujone treatment showed a significant increase in CTL production in both the in vivo and in vitro models, as indicated by a significant increase in the life span of tumor bearing animals. All these results indicate that administration of Thujone could enhance the immune response of mice. There was a significant reduction in solid tumor development, mediated by the presence of alert immune responses during Thujone administration.