Genetically encoded probes powerfully and non-invasively target specific tissues, cells, and subcellular locations. iRFP, a novel near-infrared fluorescent protein with low quantum yield whose absorption and fluorescence maxima are located at wavelengths longer than the Q-band of hemoglobin absorption, is ideal for PAT. Here, we report on an in vitro comparison of iRFP with other far-red fluorescent proteins, and its use in imaging a mouse tumor xenograft model. In an in vivo experiment, we stably transfected iRFP into MTLn3 adenocarcinoma cells and injected them into the mammary fat pad of female SCID/NCr mice, then imaged the resulting tumors two and three weeks post injection. The contrast increase from the protein expression was high enough to clearly separate the tumor region from the rest of the animal.