  We study the effect of electron-electron interactions in the optical
conductivity of graphene under applied bias and derive a generalization of
Elliot's formula, commonly used for semiconductors, for the optical intensity.
We show that {\it excitonic resonances} are responsible for several features of
the experimentally measured mid-infrared response of graphene such as the
increase of the conductivity beyond the "universal" value above the Fermi
blocked regime, the broadening of the absorption at the threshold, and the
decrease of the optical conductivity at higher frequencies. Our results are
also in agreement with {\it ab initio} calculations in the neutral regime.
