The green-fluorescent proteins (GFP) are a unique class of proteins involved in bioluminescence of many cnidaria. The GFPs serve as energy-transfer acceptors, receiving energy from either a luciferaseoxyluciferin complex or a Ca2+-activated photoprotein, depending on the organism. Upon mechanical stimulation of the organism, GFP emits green light spectrally identical to its fluorescence emission. These highly fluorescent proteins are unique due to the nature of the covalently attached chromophore, which is composed of modified amino acid residues within the polypeptide. This report describes the characterization of the Aequorea victoria GFP chromophore which is released as a hexapeptide upon digestion of the protein with papain. The chromophore is formed upon cyclization of the residues Ser-dehydroTyr-Gly within the polypeptide. The chromophore structure proposed here differs from that described by Shimomura [(1979) FEBS Lett. 104, 220] in a number of ways.