A study is made of the haemolytic-rate curves of saponin and the ionic detergents, and of the effect on the curves of haemolytic accelerators. An equation relating percentage haemolysis and time is derived for a collection of identical cells assuming that the lytic agent interacts with the cell wall in two main stages, which are believed to be the penetration of the lysin into the cell wall and the breakdown of the membrane lipoprotein-cholesterol complex. The agreement between the theoretical and experimentally observed curves justifies these assumptions, and indicates that the overall haemolytic rate depends on the concentration of haemolytic agent adsorbed on the erythrocyte walls. The results suggest that the above lytic agents haemolyze by the same general mechanism. The effect on the curves obtained by introducing a heterogeneous collection of cells is considered. The effect of the haemolytic accelerators is found to be proportional to their free energy of adsorption at the oil/water interface. These substances seem to have a general weakening effect on the cell wall; they assist the breakdown of the membrane lipoprotein-cholesterol complex which constitutes one of the main stages in the haemolysis.