AbstractMIP26/AQP0 is the major lens fiber membrane protein and has been reported to interact with many other lens components including crystallins, lipid, and cytoskeletal proteins. Regarding crystallins, many previous reports indicate that MIP26/AQP0 interacts with either only α‐crystallin or some specific γ‐crystallins. Considering the possibly important role of MIP26/AQP0 in the reduction of light scattering in the lenses, we have further investigated its interaction with crystallins using confocal fluorescence resonance energy transfer (FRET) microscopy. Specifically, we used MIP26 tagged with a green fluorescence protein (GFP) as a donor and a crystallin (αA‐, αB‐, βB2‐, or γC‐crystallin) tagged with a red fluorescence protein (RFP) as an acceptor. The two plasmids were cotransfected to HeLa cells. After culture, laser scattering microscopy images were taken in each of the three channels: GFP, RFP, and FRET. The net FRET images were then obtained by removing the contribution of spectral bleed‐through. The pixels of net FRET were normalized with those of GFP. The results show the presence of measurable interactions between MIP26 and all crystallins, with the extent of interactions decreasing from αA‐ and αB‐crystallin to βB2‐ and γC‐crystallin. Competitive interaction study using untagged αA‐crystallin shows decreased net FRET, indicating specificity of the interactions between MIP26 and αA‐crystallin. We conclude that all crystallins interact with MIP26, the physiological significance of which may be a reduction in the difference of refractive index between membrane and cytoplasm. J. Cell. Biochem. 104: 51–58, 2008. © 2007 Wiley‐Liss, Inc.