Hemocyanins are high molecular weight oxygen-carrying proteins that occur in the molluscs and arthropods. The oxygen-binding site in these proteins is a pair of copper atoms bound directly to ammo acid side chains. The biscopper sites of these proteins bind single molecules of dioxygen or carbon monoxide. In arthropods there are two copper atoms per approximately 70 000 daltons of protein. This corresponds to the molecular weight of the minimum polypeptide chain. In molluscs however there are two copper atoms per 50,000 daltons of protein. This does not correspond to the minimum polypeptide chain although it does correspond to the minimal functional unit. The minimal polypeptide chain in molluscan hemocyanins is approximately 400,000 daltons and appears to be composed of eight or more 50 000 dalton units linked together like a string of pearls. In the molluscs, the native hemocyanins found in the hemolymph generally occur as giant cylindrical molecules 350 Å in diameter, 380 Å long, with molecular weights of 9 000,000. These large molecules are composed of approximately 20 polypeptide chains. In the hemolymph of the arthropods depending upon the species, hemocyamins occur as hexamers dodecamers 24-mers and 48-mers. The molecular weight of the 48-mers is about 3 600,000. The respiratory functions of hemocyamns show a wide range of allosteric properties. The large molecules commonly show cooperativity in oxygen binding. The number of interacting subunits is in some cases dependent upon external conditions of pH and ionic composition. The oxygen affinity is usually sensitive to pH and to low molecular weight cofactors like chloride, calcium and magnesium ions. The intrinsic oxygen binding properties of an organism s hemocyanin and its allosteric control by modulators allow organisms possessing these giant molecules to adapt to their environmental conditions.