A single dose of thiopental (37-42 mg/kg) sufficient to achieve anesthesia induction increased the growth rate of a 3-methylcholanthrene-induced syngeneic murine fibrosarcoma in C57B1/6 mice. While no alterations in in vitro growth kinetics of tumor cells cultured with thiopental could explain these data, significant alterations in cell-mediated immunity were observed. Spleen cells from C57B1/6 mice treated with thiopental were impaired in their ability to respond in a mixed leukocyte culture (MLC) to BALB/c stimulator cells. Spleen cells from thiopental-treated mice suppressed the ability of cells from untreated animals to respond in a third-party MLC. The degree of suppression was directly proportional to the number of cells from thiopental-treated mice in the third party. When these cells were added to an MLC at various times during the culture period, suppression of the early phases of the MLC was seen. Removal of plastic adherent cells from the third party resulted in loss of suppression, but treatment of third-party cells with anti-thy 1.2 serum or irradiation did not prevent suppression. Transfer of spleen cells from thiopental-treated mice enhances growth of the B-16 melanoma. Suppressor-cell activity may be one mechanism by which thiopental promotes tumor growth.