We describe a method for producing polyclonal antibodies against peptide antigen cytochrome P450 1A2 and 3A4 using a tandem repeat of the epitope region and incorporation of proline residue between the repeated sequences. An ELISA assay revealed more efficient generation of polyclonal antibodies to tandem repeat peptide antigens than mono-epitope peptides. The incorporation of proline residues further stimulated antibody production.