Killing of tumor cells by lymphoid cells is important in cell-mediated immunity and defense against cancer. The authors determined that halothane, invitro, inhibits the killing of YAAC-1 ascites tumor cells from A/jax mice by sensitized peritoneal exudate cells from C57/Black/6 mice. Lysis of tumor cells was quantitated by release of51Cr into the culture medium. Inhibition of cell-mediated cytotoxicity ranged from 3 per cent in 0.5 per cent halothane to 44.7 per cent in 2.5 per cent halothane and was related to the duration of halothane exposure. A 12 per cent inhibition of cytotoxicity by 80 per cent nitrous oxide was not statistically significant, but was of a magnitude near that of an equipotent concentration of halothane. The inhibition of cytotoxicity by halothane and nitrous oxide observedin vitromay partially account for the inhibition of cytotoxicity observed when patients undergo surgical operation.