  We give a microscopic derivation of the Clausius-Mossotti relations for a
homogeneous and isotropic magneto-dielectric medium consisting of radiatively
broadened atomic oscillators. To this end the diagram series of electromagnetic
propagators is calculated exactly for an infinite bi-cubic lattice of
dielectric and magnetic dipoles for a lattice constant small compared to the
resonance wavelength $\lambda$. Modifications of transition frequencies and
linewidth of the elementary oscillators are taken into account in a
selfconsistent way by a proper incorporation of the singular self-interaction
terms. We show that in radiatively broadened media sufficiently close to the
free-space resonance the real part of the index of refraction approaches the
value -2 in the limit of $\rho \lambda^3 \gg 1$, where $\rho$ is the number
density of scatterers. Since at the same time the imaginary part vanishes as
$1/\rho$ local field effects can have important consequences for realizing
low-loss negative index materials.
