  Disperion relation of EM field in a chiral medium is discussed from the
viewpoint of constitutive equations to be used as a partner of Maxwell
equations. The popular form of Drude-Born-Fedorov (DBF) constitutive eqs is
criticized via a comparison with the first-principles macroscopic constitutive
eqs. The two sets of eqs show a decisive difference in the dispersion curve in
the resonant region of chiral, left-handed character, in the form of presence
or absence of linear crossing at k=0. DBF eqs could be used only as a
phenomenology in off-resonant region.
