Fluorescent protein (FP) has enabled the analysis of biomolecular interactions in living cells, and bimolecular fluorescence complementation (BiFC) represents one of the newly developed imaging technologies to directly visualize protein-protein interactions in living cells. Although 10 different FPs that cover a broad range of spectra have been demonstrated to support BiFC, only Cerulean (cyan FP variant), Citrine and Venus (yellow FP variants)-based BiFC systems can be used under 37 degrees C physiological temperature. The sensitivity of two mRFP-based red BiFC systems to higher temperatures (i.e., 37 degrees C) limits their applications in most mammalian cell-based studies. Here we report that mLumin, a newly isolated far-red fluorescent protein variant of mKate with an emission maximum of 621 nm, enables BiFC analysis of protein-protein interactions at 37 degrees C in living mammalian cells. Furthermore, the combination of mLumin with Cerulean- and Venus-based BiFC systems allows for simultaneous visualization of three pairs of protein-protein interactions in the same cell. The mLumin-based BiFC system will facilitate simultaneous visualization of multiple protein-protein interactions in living cells and offer the potential to visualize protein-protein interactions in living animals.