Four hours after the administration of halothane to phenobarbital-pretreated rats, subcellular fractions of liver were isolated and the proteins in the fractions were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, transferred to nitrocellulose sheets, and immunochemically stained with anti-trifluoroacetylated antibodies. The microsomal fraction contained the highest level of trifluoroacetylated adducts. Its major trifluoroacetylated component was immunochemically identified as a phenobarbital-inducible form of cytochrome P-450 (54 kDa), whereas the other observed trifluoroacetylated protein fraction (59 kDa) was not identified. The plasma membrane fraction also contained a 54-kDa trifluoroacetylated adduct, which was immunochemically related to the 54-kDa cytochrome P-450. Microsomes from untreated rats that were administered halothane contained only the 59-kDa trifluoroacetylated protein fraction. The specificity of the immunochemical staining for the bound oxidative metabolite of halothane was confirmed by the finding that rats treated with deuterated halothane had considerably less stained liver proteins than did those treated with halothane. These results suggest that the CF3COX oxidative metabolite of halothane is so reactive that it binds predominantly to the cytochrome P-450 that produced it.