Interactions of heparin and protamine in fresh human serum, in amounts far below those required for complement depletion by either agent alone, were found to induce virtually complete depletion of total hemolytic complement activity. This depletion was dependent on time, temperature, pH, divalent cations, and serum concentration. The predominant complement component hemolytic activity depleted was C1; under appropriate reaction conditions C4 and C2 were depleted as well. Equivalent amounts of heparin along induced lesser but substantial depletion of C3-9, whereas equivalent amounts of protamine had no effect upon complement component activities. We conclude that interaction of heparin with protamine, like interaction of antibody with antigen, markedly enhances its ability to interact with the first component of complement and activate the classical complement pathway. It is suggested that complement activation by interactions between certain polyanions and polycations, like interactions between antigens and antibodies, may have a role in the initiation of inflammatory reactions.