Flavin-containing monooxygenases (FMOs) catalyze the oxidation of a diverse array of xenobiotic compounds. The purpose of this investigation was to develop a specific immunological probe to human hepatic flavin-containing monooxygenase (HLFMO3). An oligopetide corresponding to amino acid residues 257-270 of HLFMO3 was coupled to keyhole limpet hemocyanin (KLH) through the sulfhydryl group of a cysteine residue added to the amino-terminus of the peptide. This peptide-KLH conjugate was used to generate a polyclonal antibody. The resulting immunoglobulin showed specific Western blot reactivity with HLFMO3 protein in human hepatic microsomes, the same protein that is recognized by a polyclonal antibody directed against macaque liver FMO. These findings demonstrate that an antibody directed against a synthetic peptide derived from HLFMO3 can be easily produced in large quantities and used in studies for the immunodetection and immunoquantification of HLFMO3. This is also the first antipeptide antibody directed against an FMO of any species.