Trifluoroacetate (TFA), a common metabolite of halothane and fluroxene, conjugated to human and guinea pig albumin induces specific delayed-type hypersensitivity in guinea pigs. Histologic examination of cutaneous hypersensitivity reactions characterized by erythema and induration 24–48 hours after intradermal challenge with antigen revealed perivascular mononuclear infiltration. These findings support the hypothesis that trifluoroacetate, a metabolite of halothane and fluroxene, can serve as a hapten to elicit specific cellular immune responses.