ICln is a highly conserved, ubiquitously expressed protein, mainly localized in the cytosol as a water-soluble protein but partly found also within or close to the cell membrane. It plays a critical role in regulatory volume decrease and is believed to act as an anion channel during cell swelling, when is translocated from the cytosol to the plasma membrane. It was shown that the protein forms functional ion pores after reconstitution into artificial bilayers. The putative channel model predicts the association of at least two ICln molecules to form a functional ion-conducting pore. Truncated ICln (ICln159) lacking the third acidic domain (AD3) and major parts of the C-terminus did not form homomultimers as shown with NMR. In this study we investigated ICln-selfinteractions in living cells using the FRET technique. ECFP and EYFP were used as donor and acceptor dye, respectively and fused either to the C-terminus (ICln-CFP/ICln-YFP), N-terminus (CFP-ICln/YFP-ICln) or both C- and N-terminus (CFP-ICln-YFP) and (co)expressed in NIH3T3 fibroblasts. FRET was assessed from the increase of CFP-fluorescence after 2 minutes of acceptor photobleaching. FRET signals were detected in cells expressing CFP-ICln-YFP, indicating C-N-terminal intramolecular and/or intermolecular interactions. FRET was also evident in cells expressing ICln-CFP/ICln-YFP but not when using CFP-ICln/YFP-ICln, indicating that intermolecular interactions occur between two or more ICln molecules with their C-termini, but not with their N-termini. Cells expressing CFP-ICln159/YFP-ICln159 shown strong FRET, indicating that the absence of the C-termini allows N-terminal interactions. The interaction between C-terminus, investigated using ICln159-CFP/ICln159-YFP, showed weak FRET-signals. No FRET signals could be measured neither between the C- nor between the N-termini, using ICln truncated at position 134 (ICln134) lacking AD3 and AD2, indicating that the presence of AD2 is necessary for ICln-ICln-interactions in living cells.