Red cells sensitized with a complement-binding antibody and then incubated with fresh serum have been shown to be coated with β1C and β1E globulin, two components of the complement system that have been isolated recently. Red cells presumably in the state EAC′1,4 reacted with anti-β1E, and cells presumably in the state EAC′1,4,2,3a reacted with anti β1E and with anti-β1C. Agglutination of complement-coated cells by `broad spectrum' antiglobulin sera was effectively inhibited by purified β1E and β1C globulin. Red cells from certain patients with acquired haemolytic anaemia were found to be coated in vivo with β1E and β1C globulin. The function and significance of the two serum components have been discussed.