There is provided a skin test diagnostic reagent comprising at least one CFP-10 epitope polypeptide, at least one ESAT-6 epitope polypeptide and at least one Rv3615c epitope polypeptide, the reagent eliciting a positive result when administered in a skin test to an animal infected with Mycobacterium bovis or Mycobacterium tuberculosis. The reagent is useful in that it can be used in a method to determine whether an animal is infected with Mycobacterium bovis or Mycobacterium tuberculosis.