The kinetics of haemolysis of human red blood cells by synthetic anionic detergents has been studied. It is shown that these detergents can destroy red cells by two mechanisms, which normally operate simultaneously, one leading to a rapid destruction of the cells whilst the other is a slow process. The rapid process involves the action of the detergent on free phospholipid in the cell wall. This component is easily removed from the cell membrane, and cells thus treated will not undergo rapid haemolysis. The slow process is identical with that by which most haemolytic agents (saponins, nonionic detergents and butanol) act, and if it alone can operate yields percentage haemolysis-time curves which are sigmoid in shape. This process takes place in several stages and seems to involve the slow breakdown of a lipoprotein complex on the cell surface. If phospholipid is removed from the cell wall, it is slowly replaced from phospholipid bound as lipoprotein or possibly by synthesis from metabolites in the membrane. The equilibria which maintain the structure of the cell wall are discussed.