A protein, aequorin, having no more than the usual, slight fluorescence of proteins in general, gives a bright, blue, light-emitting reaction with CaZL, leading irreversibly to a strongly blue fluorescent protein product; the reaction mechanism presumably involves an unusual intramolecular chemical change catalyzed by Ca2+. A functional moiety in this bioluminescence reaction can be separated from the protein in the form of either of two blue fluorescent compounds designated as "AF-350" and "AF-400." The former has an absorption maximum at 350 mp, and molecular weight of 277 by mass spectrometry; its separation from aequorin is by treatment with urea and mercaptoethanol at alkaline pH, leaving the protein moiety as "apoaequorin-SH." The latter fluorescent compound (AF-400) has an absorption maximum at 400 mp; its separation from aequorin is by treatment with NaHS03, leaving apoaequorin-SO. The chemical composition of aequorin includes, in addition to the above-mentioned functional moiety, 18 different amino acids, an unidentified other amino compound, glucose, no phosphate, and no acetylneuraminic acid.