The mechanism of the precipitin reaction between antipneumococcus sera and the type-specific soluble carbohydrate is investigated. The sensitivity of the reaction is found to be generally constant when expressed by the product of the concentrations of the two reacting substances. Precipitin index (P.I.) is defined as one-millionth of the reciprocal value of this product. See PDF for Structure Zonal phenomena and their bearing on the absolute concentration and the equivalent weight of the antibody are discussed. The greater tendency towards exhibition of a post-zone in Type III is connected with the lower acid equivalent of the homologous specific carbohydrate. The influence of the addition of normal serum and the influence of slight changes in pH are studied. The addition of normal serum as well as the increase in pH promote the pro-zone, whereas decrease in pH promotes the post-zone. The sensitivity of the reaction is accordingly influenced by the pH in different ways depending on the range of concentration. The precipitin index allows the recognition and elimination of zonal irregularities. Thus it offers a method for the standardization of pneumonia antibody.