1. The molecular quantities of hemolysin and complement combining component or components (C'1) involved in hemolysis have been calculated on the basis of new, quantitative, absolute methods of analysis.
2. Molecular combining ratios between antigen, antibody, and C'1 have been established.
3. The data are shown to be in accord with the theory of combination of multivalent antigen with multivalent antibody.
4. The fixation of complement by antigen-antibody combination is qualitatively and quantitatively accounted for on this basis.