The oxygen binding properties of haemocyanin from Levantina hierosolima in the absence and in the presence of calcium ions were studied. A fluorometric method for the determination of saturation curves, based on the difference in emission intensities of the reduced and oxygenated protein, was developed. In divalent ion-free solutions, the binding was found to be of the simple type. In the presence of calcium ions interaction between the binding sites takes place. Thermodynamic quantities for the oxygen binding reaction in the two cases were determined. Corresponding values for the interaction among the oxygen binding sites were also calculated. The binding behaviour of the oxygenated haemocyanin in calcium-containing solutions was found to be identical to that of haemocyanin in the absence of the ion. The findings show that, as is the case in the haemoglobin system, deoxyhaemocyanin has a lowered affinity towards oxygen; oxygenation returns the molecule to the high affinity characteristic of the calcium-free protein.