Green fluorescent protein (GFP) has been of interest to researchers recently due to its use in nonlinear microscopy and biomedical applications. Through the use of nonlinear characterization, GFP has been previously shown to exhibit saturable nonlinear absorption. This nonlinear saturation suggests the possibility of a two-photon induced population inversion in GFP, which can lead to the use of GFP as a frequency upconversion laser material, particularly for bioMEMS devices. Here, we explore this population inversion and report on the ultrafast two-photon pumped lasing properties of GFP. Specifically, we report on the lasing threshold, spectral features and output-input energy characteristics. This is the first demonstration of ultrafast two-photon pumped lasing in a biological chromophore. Furthermore, GFP also appears to have a nearly cubic dependence of the saturation intensity on the concentration of GFP molecules. In this report, we also investigate this relationship by looking at the theoretical components affecting the saturation intensity to determine their dependence on GFP concentration.