Protein function is often mediated by the formation of stable or transient complexes. Here we present a method for testing protein-protein interactions in plants designated bimolecular fluorescence complementation (BiFC). The advantages of BiFC are its simplicity, reliability, and the ability to observe protein-protein interactions in different cellular compartments including membranes. BiFC is based on splitting the yellow fluorescent protein (YFP) into two nonoverlapping N-terminal (YN) and C-terminal (YC) fragments. Each fragment is cloned in-frame with a gene of interest, enabling expression of a fusion protein. Reconstitution of the fluorescing YFP chromophore takes place upon interaction of protein pairs that are coexpressed in the same cells.