We have shown that the oxygen binding and association-dissociation equilibrium of Callianassa hemocyanin can be interpreted in terms of a single allosteric process. We intend to examine the nature of this allosteric change, and to investigate the relationship between the thermodynamics of oxygen binding and the thermodynamics of the association reaction. We will also study the kinetics of the latter process. Further, we shall continue studies of the subunit structure of molluscan hemocyanins, which appear to possess the largest polypeptide chains observed to date in any protein (about 4 x 10 to the 5th power daltons). We wish to determine if Busycon hemocyanin is composed exclusively of such chains, or involves other components as well.