Two-photon absorption is a nonlinear phenomenon that is observed in materials irradiated with the combined energy of two photons, which matches the transition energy between the ground state and the excited state. Molecules exhibiting strong two-photon absorption have great potential in a wide range of applications such as three-dimensional fluorescence imaging, optical data storage, and microfabrication.(9.6) Green fluorescent protein (GFP) can be excited by a two-photon mechanism and exhibits a nonlinear behavior of the effective two-photon absorption coefficient. Here we present the quantitative determination of the two-photon absorption saturation of GFP in both a homogeneously broadened and inhomogeneously broadened system. We further show that, dominated by strong nonlinear saturation, GFP has a large intrinsic two-photon absorption cross section (sigma (TPA)) value that is several orders of magnitude larger than that of chemically synthesized compounds.