To study the mechanism of haemolysis in G6PD deficient erythrocytes, studies were undertaken in G6PD deficiency and in normal erythrocytes artificially loaded with calcium. Significantly increased concentrations of calcium, calcium activated neutral protease (CANP) and calcium ATPase were found in patients of G6PD deficiency. However, the membrane bound calcium, the total glycoprotein and sulphydryl groups of membrane were observed to be decreased. Similar results were also observed in the normal erythrocytes when loaded with calcium. These results point to the role of the proteolytic process in membrane modification, and altered membrane permeability during the haemolytic process. Our observations in G6PD deficiency and in in vitro point to the existence of a calcium dependent proteolytic preconditioning of erythrocyte accelerating the haemolysis.