  A seemingly obvious extension of the weak equivalence principle, in which all
matter must respond to Post-Newtonian gravitational fields, such as
Lense-Thirring and radiation fields, in a composition-independent way, is
considered in light of the Kramers-Kronig dispersion relations for the linear
response of any material medium to these fields. It is argued that known
observational facts lead to violations of this extended form of the equivalence
principle. (PACS numbers: 04.80.Cc, 04.80.Nn, 03.65.Ud, 67.40.Bz)
