In experiments that have become classic in bioluminescence, Dubois first prepared from a luminous elaterid, Pyrophorus, and a luminous clam, Pholas, respectively crude extracts containing a substrate, luciferin, and an enzyme, luciferase, which luminesced on mixing in aqueous solution containing dissolved oxygen.In the years that have followed, efforts have repeatedly been made to separate functionally similar components from numerous other, diverse types of luminous organisms Although the majority of these efforts have proved unsuccessful, about a dozen biologically specific, chemically different uciferin-luciferase systems have by now been obtained in varying degrees of purification. The present investigation has resulted in the discovery of a new type of luminescent system, differing from those hitherto extracted in being comprised of a single organic component, with the properties of a protein. In aqueous solution either devoid of, or saturated with, oxygen, this protein gives a light-emitting reaction on the addition of Ca²⁺. With the procedures employed, nearly 10,000 individual specimens of the hydro-medusan, Aequorea, yielded about 5 mg of the highly purified, active substance which we have named "Aequorin."