INTRODUCTIONThe phospholipid phosphatidylethanolamine (PE) can be conjugated via its head group to a number of fluorophores, including rhodamine, BODIPY (boron-dipyrromethene; 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene), and NBD (N-(7-nitrobenz-2-oxa-1,3-diazole-4-yl)). These probes can be used to label biological membranes and to study phospholipids within membranes. Rhodamine-PE, which does not readily exchange between lipid bilayers, has also been used in membrane fusion assays. Rhodamine-DHPE (lissamine rhodamine B 1,2-dihexyldecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium salt) and NBD-PE have been used together in resonance energy transfer studies in membranes. This protocol describes the labeling of membranes with fluorescent PE.