Bimolecular fluorescence complementation (BiFC) is a technique to visualize protein-protein interactions in living cells, and has been widely used in various model organisms. The principle of the BiFC assay is based on the reconstitution of an intact fluorescent protein. The two non-fluorescent fragments are fused to proteins of interest that may interact. If the two proteins interact, the two non-fluorescent fragments are brought together to reconstitute an intact fluorescent protein. The purpose of this protocol is to calculate signal-to-noise (S/N) ratio in the bimolecular fluorescence complementation (BiFC) assay and to provide a semi-quantitative analysis of protein-protein interaction (PPI) in living cells.