Rabbit gamma globulin has been extensively reduced, and its polypeptide chains separated by gel filtration on Sephadex G-200 in SM guanidine hydrochloride. The larger H (or A) chains make up two-thirds of the molecule and have a molecular weight of approximately 55,000 each. The smaller L (or B) chains account for the other one-third and have a molecular weight of approximately 25,000 each. The data are consistent with a model of the gamma globulin molecule that has two H and two L chains.