proportion to the concentration of peroxidase present. Whether luciferase is an oxidizing enzyme made up of an albumin associated with some heavy metal as iron, copper or manganese is uncertain. From analyses of whole _Cypridina_, kindly made for me by Prof. A. H. Phillips of Princeton University, all three of these metals, which we know to be associated with biological oxidations, are present, and it is quite possible that one of them is concerned with the oxidation of luciferin. Although I have tested a great many oxidizers, organic and inorganic, and a large number of oxidizing enzymes from blood and tissue extracts of animals rich in iron, copper and manganese, I have found no material which is capable of taking the place of _Cypridina_ luciferase. Peroxidases or oxidases of plants, hæmoglobin, hæmocyanin, extracts of mussels, manganese containing blood of various marine crustacea and mollusks will give no light on mixing with luciferin. Such active oxidizers as KMnO_{4}, H_{2}O_{2}, BaO_{2}, and many others, will not oxidize _Cypridina_ luciferin with light production, although they can oxidize _Pholas_ luciferin with light production. The action of _Cypridina_ luciferase is very highly specific. It is found only in the luminous organ of _Cypridina hilgendorfii_, not in non-luminous parts and not in a non-luminous species of _Cypridina_ closely related to _hilgendorfii_. Luciferins and luciferases from closely allied luminous forms will mutually interact to produce light, but no light appears if these substances come from distantly related forms. Thus firefly (_Photuris_) luciferin will give light with _Pyrophorus_ luciferase and _vice versa_, but _Cypridina_ luciferin will give no light with firefly (_Luciola_) luciferase or _vice versa_, nor with _Pholas_ luciferase or _vice versa_. The faint luminescences sometimes observed on mixing firefly or _Cypridina_ luciferase with boiled extracts of non-luminous forms, or of distantly related luminous forms, are probably caused by photophelein in the boiled extract. Like the plant peroxidases, _Cypridina_ luciferase is not readily affected by the action of chloroform, toluol, etc. Unlike the plant peroxidases, it will not oxidize (_i.e._, produce coloration) in either presence or absence of H_{2}O_{2}, any of the hydroxyphenol or aminophenol compounds, such as pyrogallol, a-naphthol, para-diamino-benzine, gum guaiac, etc., commonly used as peroxidase reagents. Neither will luciferase produce light with any substances, such as oils, lophin, pyrogallol, gallic acid, esculin, etc., which we know to be capable of oxidation with light production by other means. The luciferases are very highly specific and act only upon the luciferins of the same or closely related species. They must be placed by themselves in a new class of oxidizing enzymes.