A luciferase intermediate in the bacterial [Photobacterium fischeri and Beneckea harveyi] bioluminescence system, which is formed by reaction of enzyme with FMNH2 and O2 is shown to emit light with added aldehyde under anaerobic conditions. The reaction with O2 is thus effectively irreversible under the conditions used. The flavin chromophore has an absorption maximum at about 370 nm and the potential activity (bioluminescence yield) in the further reaction of the isolated intermediate with aldehyde is strictly proportional to the amount of this flavin chromophore.