1. Highly purified preparations of homogeneous antibody can be made by the salt dissociation methods (6, 7) without any change in sedimentation due to the method of purification. 2. Antibodies prepared from sera of various animal species fall into two groups as regards molecular weight; in one group cow, horse, and pig, a heavy molecule of molecular weight 990,000 is formed; in human being, rabbit, and monkey, the molecular size is that of the normal γ serum globulin. Both types of antibody molecules are either not compact or not spherical since the frictional ratios f/f0 are 2.0 and 1.5 respectively. 3. Horse antibody shows an unchanged activity and sedimentation diagram between pH 3.44–9.06, although there is some aggregation at the more acid and some dissociation at the more alkaline pH. At pH 1.44 the antibody activity is unchanged but some breakdown of the molecule takes place. At pH 12.4 activity is destroyed and the molecule is completely broken down. 4. Some horse antibody preparations show evidence of breakdown of the antibody into inhomogeneous material on continued immunization over a long period.