The combined use of fluorescence resonance energy transfer (FRET) microscopy and expression of genetic vectors encoding protein fusions with green fluorescent protein (GFP) and blue fluorescent protein (BFP) provides an exceptionally sensitive method for detecting the interaction of protein partners in living cells. The acquisition of FRET signals from GFP- and BFP-fusion proteins expressed in living cells was demonstrated using an optimized imaging system and high sensitivity charge coupled device camera. This imaging system was used to detect energy transfer signals from a fusion protein containing GFP physically linked to BFP expressed in living HeLa cells. In contrast, the co-localization of noninteracting GFP- and BFP-fusion proteins was not sufficient for energy transfer. The FRET imaging system was then used to demonstrate dimerization of the pituitary-specific transcription factor Pit-1 within the living cell nucleus.