3,3′-Dioctadecyloxacarbocyanine perchlorate is a green fluorescent lipophilic tracer, which is weakly fluorescent in water but highly fluorescent and quite photostable when incorporated into membranes. In Vitro: A fluorescent dialkyl carbocyanine dye (DiOC18(3)) is used to label muscle membranes, and the membranes accessible to the dye were observed by confocal laser scanning microscopy. A loss of cell membrane integrity results from contractile activity, enabling DiOC18(3) staining of internal membranes. The resulting staining pattern is striking and fibers with damaged cell membranes are easily distinguished from uninjured ones^[1]. 3,3′-Dioctadecyloxacarbocyanine perchlorate is a cationic oxacarbocyanine dye. The absorption and fluorescence spectra of 3,3'-dioctadecyloxacarbocyanine, a cationic oxacarbocyanine dye have been studied in aqueous and nonaqueous media containing egg phosphatidylcholine (PC) as well as in different solvents of diverse nature. The results show the evidence of complex formation of the dye in the ground and in the excited states with PC. The excited state interaction of the dye with PC suggests the electron transfer from PC to dye and this is supported by photovoltage generation in a photoelectrochemical cell consisting of dye and PC in aqueous medium^[2].