We present an ab initio study of the optical absorption properties of a particularly interesting fluorescent protein (E 2 GFP), whose complex photophysics still escapes elucidation. In particular, we focus on the role of the protein environment, showing that the effects of both nearby residues and the external field due to residues not accounted for explicitly are needed to properly reproduce the experimental data. The spectra calculated taking such contributions into account provide for the first time a robust identification of the states relevant for the photophysics of this system.