The emissive properties of reduced and oxygenated hemocyanin from Levantina hierosolima at neutral and alkaline pH were studied. A large enhancement in fluorescence intensity is observed on complete reduction, fourfold at neutral pH. In partially reduced protein the enhancement is proportional to the fraction reduced. The quenching upon oxygen binding is traced to radiationless energy transfer from the tryptophanyl residues to the Cu⃛O groups. Application of Förster's theory leads to a value of 25 Å for the distance between the donor and the acceptor in the “equivalent oscillators system,” defined as a single donor–acceptor pair with parallel orientation which would display the same emissive properties as the actual protein molecule. From a comparison of the data obtained at neutral and alkaline pH, it is concluded that the internal arrangement of the donor–acceptor system is not affected by the extensive dissociation which the molecule undergoes in the transition between the two pH values considered. The findings indicate that the fluorimetric method provides a sensitive tool for the study of oxygen binding equilibria in hemocyanin.