1. There is serological evidence that ragweed pollen antigen contains a factor which causes an increased hemolysin titre for sheep cells, when injected into rabbits.
2. It remains questionable whether the antigen absorbs normal rabbit hemolysins. It does, however, absorb anti-ragweed hemolysins, as demonstrated by hemolysis inhibition test.
3. Dilution and lyophilization (Mudd-Flosdorf method) of antigen-antibody mixture shows that one hemolytic unit of anti-ragweed rabbit serum combines with 0.00007 mg. of low ragweed total N. There were three zones of inhibition of hemolysis demonstrated.
4. The immune hemolysins are completely absorbed by sheep cells and by Forssman antigen, but not by human cells of groups A or B.
5. Although Forssman antigen was able to absorb anti-ragweed hemolysin, the ragweed antigen was not able to absorb Forssman antibody.
6. Human ragweed sensitive sera from 22 cases with hay fever, before treatment, did not show increased titre of heat-labile or heat-stable hemolysin. There was no change after treatment.
7. Among the human cases there were encountered blood groups A, B, and O. Heat-labile hemolysins for sheep cells were present in nineteen of the twenty-two sera regardless of blood groups.