Quantitative Experiments with Antibodies to a Specific Precipitate. I

1. Rabbits were injected with the washed specific precipitate from Type II antipneumococcus horse serum. Antibody in the resulting antiserum was determined by the quantitative agglutinin method using various specific precipitates as antigens. 2. Suspensions of Types I and II antipneumococcus horse specific precipitates, as well as the specific precipitates derived from Type VIII Pn (anti-C portion), and H. influenzae horse antisera were found to remove the same amount of antibody from the immune rabbit serum. 3. Purified antibody solutions prepared by dissociation methods from Types I and II antipneumococcus horse sera were found to remove the same quantity of antibody as did the homologous specific precipitates. 4. Specific precipitates from anti-crystalline egg albumin and anti-diphtheria horse sera were found to remove only a fraction of the antibody. The reasons for this are discussed. 5. A specific precipitate prepared from pepsin-digested Type I anti-pneumococcus horse serum removed all of the antibody to the homologous antigen from the rabbit anti-precipitate serum, but followed a different quantitative course. 6. From the quantitative course of these reactions and from experiments with specific precipitates from anti-Pn rabbit and pig sera it is concluded that the only antigenic specificity demonstrable for the antibodies investigated was that due to their common origin, and that the groupings responsible for their antibody function constitute either a small part of the total protein molecule or else are non-antigenic.