Experimental evidence has been obtained for the transmembrane-peptide-induced clustering of phospholipids in the fluid bilayer state by use of the nearest-neighbor recognition method. Specifically, the exchangeable phospholipid monomers of 1a and 1b were found to mix, apparently, nonideally and cooperatively in liposomal membranes containing 9 mol % of the nonchannel form of gramicidin A. Equilibrium dimer distributions that were measured as a function of the initial ratio of 1a/1b support a model in which discrete clusters are formed that contain two molecules of 1b.