GFP has been mainly developped for cellular localization of proteins (see Technical focus, TIGS, 1995, 11: 320). Moreover, the engineering of different GFP mutants with altered fluorescence characteristics allows colocalization of different proteins (Heim and coll. PNAS,1994,91:12501). The FRET phenomenon, that we are developping, consists in a fluorescence transfer between a donor and a receptor fluorochromes. If excitation and emission wavelengths are compatible, the FRET is easily measurable. The main parameter of the reaction is the distance between donor and receptor, which must be in the range of nanometers. This is precisely the kind of values in protein-protein interactions. Using the Y66H GFP mutant as donor and the S65T as receptor, we constructed fusions between these two proteins. Analysis of the spectra of these double molecules compared with a mix of the two independantly produced proteins proved that FRET occurs in fact between the two fluorochromes. Moreover, separation of the two GFPs by proteolytic cleavage of the fusion product leads to FRET disappearance (see also Heim and Tsien, Current Biol., 1996, 6:178). FRET can then be used for in vitro monitoring a complex between two yeast proteins. Rvs161p has been shown by dihybrid strategy to form a complex with Rvs167p. Use of FRET for in vitro visualization of this complex is in progress.