The role of antigen, antibody, and specific complement components in cytocidal, immune adherence, and phagocytic reactions is reviewed briefly. Special details on immune adherence are emphasized since antigens which are either soluble or particulate may be detected with striking sensitivity provided that the antigen-antibody complex will interact with complement. Antigens on cell surfaces which are located at sites which are not susceptible to a cidal or lytic effect may be detected either by immune adherence or by phagocytosis. A method for performing quantitative phagocytic reactions is reviewed briefly. Estimations have been made of the molecular requirements of antibody and complement components for these reactions.