Antibody and complement are the principal noncellular factors in host resistance. Antibody recognizes the foreign nature of antigenic substances in the body. Complement is a protein system which is enzymatically and sequentially activated by the combination of complement-fixing antibody with antigen. The complement system is designed to destroy the invading substance by direct destruction (cellular or bacterial lysis), by enhancing and assisting in phagocytosis, and by liberation of vasoactive substances. These actions and the present concepts of the chemistry and the reaction mechanisms of complement are discussed.