Summary: Sera from persons immunized with pneumococcal type specific polysaccharides contained factors which would fix to the skin of most injected subjects and which would block the reagin-antigen skin reaction. The factors showed specificity, had properties different from those of antigen, so were considered to be antibodies. In each case this antibody was found to be heat stable. It did not measurably coprecipitate with rabbit precipitating antiserum. The sera containing this form of antibody also contained others probably against the same antigenic component. Although precipitins were present, in these human sera, against other antigenic components, they could not be detected against the antigenic fraction corresponding to the skin-fixing, blocking antibody.