It has been shown that the "activated reactor" that is produced in certain human sera by complement activation is a stable complex of the fifth and sixth component of complement (C56). On interaction with C7, the indicator factor, a complex C567 is formed which for a short time (half-life less than 1 min) has an activated binding site and can attach itself to normal red cell membranes, conferring on them the hemolytic properties of the "heat stable" complement intermediate EC 1 approximately 7, the capacity to be lysed by C8 and C9. These cells have neither antibody nor the complement components up to C3 bound on them. The binding site-activated C567c-can similarly bind to other hydrophobic surfaces, including agarose gel where it forms a "stainable line". If the complex is not bound to a surface, the binding site decays and the resulting complex will no longer give rise to lysis. However it will still inactivate C8 and C9 in solution. The sera that can generate activated reactor apparently do so because they have an excess of C5 and C6, compared to their content of C7. The phenomenon of reactive lysis thus represents complement-mediated lysis of unsensitized cells initiated at the C5 stage by a stable complex (C56) which was generated by complement activation at a distance. The immunochemistry of the phenomenon is described and some of its implications discussed.