THE photoprotein aequorin (molecular weight about 30,000) isolated from the bioluminescent jellyfish Aequorea aequorea1–3 emits blue light (λm a x 470 nm)4 by an intramolecular reaction in aqueous solution when Ca2+ is added, in the presence or absence of molecular oxygen1. This reaction has been postulated5 to follow the scheme in (Fig. 1), in which an essential component H2O2 of aequorin is bound to the protein part in a form such as a peroxyacid or an α-hydroxyhydroperoxide which readily releases H2O2. Hypothesised roles for peroxide in aequorin (see recent review6) are consistent with this scheme. YC is a yellow chromophore of unknown structure, responsible for the colour of aequorin from which it has been isolated5, and found to be different from other components in Fig. 1.