The binding of oxygen to stripped (Ca2+,-Mg2+-free) hemocyanin from the scorpion Leirus quinquestriatus was studied over a wide range of pH and temperature. Oxygen binding was cooperative in the entire range of pH where the native hemocyanin structure is preserved. Probably 12, or even the totality of the 24 oxygen binding sites of the molecule, act as a cooperative unit. The oxygen binding affinity and the degree of cooperativity were both affected by pH. The dependence on pH of the half-saturation pressure-Bohr effect-was interpreted in terms of two oxygen-linked protons, one opposing and the other promoting oxygen binding. The effect of pH on the maximal slope of the Hill plot indicates that proton dissociation and cooperativity are linked phenomena. Analysis of the binding data using linkage theory shows that an efficient coupling of the homotropic (oxygen-oxygen) and the heterotropic (proton-oxygen) free energies of interaction can provide a satisfactory interpretation of the cooperative behavior. Our findings suggest that in arthropod hemocyanin, as in molluskan hemocyanin, cooperativity of oxygen binding can be attributed to ligand-ligand linkage. In molluskan hemocyanin, however, cooperativity is generated solely by alkaline earth ion-oxygen linkage. In arthropod hemocyanin, both alkaline earth ion-and proton-oxygen linkage can generate cooperative behavior.