The amount of the third component of complement (C3) bound to red cells of patients with the cold agglutinin syndrome was determined by a quantitative assay, measuring the fixation of the first component of complement by anti-C3. Abrupt reduction in the serum concentration of cold agglutinin by plasmapheresis markedly decreased the hemolytic rate, but the amount of C3 bound to circulating cells did not change appreciably. When this patient was transfused with normal cells. C3 accumulated on the transfused cells within 48 h to the level present on his own cells, but selective destruction of the transfused cells did not occur. When patients were subjected to acute cold stress, cell-bound C3 rose abruptly and intravascular hemolysis occurred. These studies suggest that most of the C3 detected on the circulating red cells of cold agglutinin patients is in an inactive form, and that the rate of attachment of C3 to the membrane is important in determining hemolysis.