Abstract
1. Rabbit antisera to a Type II pneumococcus specific precipitate from horse serum were tested with fractions prepared by ultracentrifugation and electrophoresis of normal and immune horse serum. 2. In one instance a rapidly sedimenting protein from normal horse serum had nearly the same quantitative antigenic properties toward the anti-antibody rabbit serum as did the purified pneumococcus antibody solutions previously reported. In another instance a comparable fraction removed only a part of the rabbit antibody. 3. Electrophoretic gamma-globulin from an immune horse serum had quantitatively the same antigenic properties as did antibody solutions prepared by salt-dissociation of specific precipitates. 4. Electrophoretic gamma-globulin from normal horse serum differed in its antigenic behavior from gamma-globulin containing antibody. The data are compared with the antigenic properties of acid and alkali treated pneumococcus specific polysaccharides toward antipneumococcus horse sera. An interpretation in terms of polymers is suggested. 5. The cross-reaction of goat serum gamma-globulin against the anti-antibody serum is reported and the extent of the reaction compared with those of goat and horse serum albumins against a rabbit antiserum to the latter.