1. The removal of the ß2A-globulins from three sera from treated ragweed-sensitive individuals by immune absorption was associated with the loss of all detectable skin-sensitizing antibody activity as demonstrated by Prausnitz-Küstner testing.
2. Gel filtration studies, with sephadex G-200, indicated that the fractions containing only macroglobulins were devoid of all detectable skin-sensitizing antibody activity.
3. The immune absorption of the γ-globulins from a serum fraction containing ß2A-globulins, γ-globulins, and a trace of ß2M-globulins had no detectable influence on the skin-sensitizing antibody activity.
4. The removal of a portion of the albumin from the allergic sera by immune absorption, with retention of the skin-sensitizing activity, indicated that the loss of skin-sensitizing antibody was not due to non-specific absorption on an antigen-antibody precipitate.
5. No inhibition of the Prausnitz-Küstner reaction was observed when sheep serum, normal human serum, or normal human γ-globulins were tested in concentrations described.
6. We conclude that the skin-sensitizing antibody activities in the three sera from treated ragweed-sensitive individuals studied were associated with the ß2A-globulins.