1. The calcium-dependency of the process of light emission has been investigated for the photoproteins aequorin and obelin. 2. The experimental curves of light production, expressed as a percentage of the maximal rate of utilisation, versus pCa are accurately predicted by the cooperative action of at least 2Ca-2+ for aequorin and at least 3Ca-2+ for obelin. 3. At low total monovalent cation concentrations, a pH change from 6.8 to 7.1 shifts the light production vs pCa curve by approx. 0.2 pCa units to the right for aequorin, while that for obelin is shifted by some 0.37 pCa units. 4. Other monovalent cations, such as Na+ are able to compete with Ca-2+ for the active sites of aequorin and also shift the light production vs pCa curve to the right. There is no apparent change in the calcium stoichiometry for light production under these conditions. 5. The same calcium stoichiometry for light emission was also obtained for aequorin or obelin in the presence of either unbuffered Ca-2+ solutions or of calcium/EGTA buffers.