On Onsager Relations and Linear Electromagnetic Materials
  We investigated the Onsager relations in the context of electromagnetic constitutive relations of linear, homogeneous materials. We determined that application of the Onsager relations to the constitutive equations relating $#P$ and $#M$ to both $#E$ and $#B$ is in accord with Lorentz reciprocity as well as the Post constraint. 
A. Lakhtakia, R.A. Depine