We have explored the use of Merocyanine 540 as a reporter fluorophore in an optical fiber sensor for general anesthetics. Depression of the freezing point of phospholipid vesicles caused by the addition of small hydrophobic molecules, such as general anesthetics, is monitored using an order-sensitive fluorophore. This reagent is immobilized at the tips of optical fibers for remote spectroscopy. Previously, the fluorophore Laurdan was shown to be a sensitive probe of lipid fluidity. However, Laurdan absorbs and emits in the ultraviolet region of the spectrum; problems associated with UV sensor operation include intrinsic/natural absorption and fluorescence, increased risk of tissue damage, low transmission in glass and plastic optical elements, and unavailability of convenient optical fiber accessories. Merocyanine 540 absorbs most strongly around 540 nm and thus eliminates many of the practical problems associated with UV operation. Preliminary studies suggest that this dye is sensitive to lipid bilayer fluidity, although the mechanisms for this sensitivity appear to be based on concentration-dependent aggregation and on the inner filter effect. Because it is highly susceptible to photobleaching, merocyanine 540 is unsuitable for this application.