The major effort in this project to elucidate the mechanism of action of calmodulin on the human erythrocyte Ca2 ion/Mg2 ion ATPase. Two lines of endeavor are being developed. The first is concerned with an improved procedure fo the isolation of calmodulin from human erythrocytes. Particular attention is being paid to the removal of a contaminant, inhibitory-like protein found in the usual preparations of calmodulin. The other line of investigation centers attention on preparations of calmodulin. The other line of investigation centers attention on the manner in which calmodulin affects the Ca2 ion/Mg2 ion ATPase activity in erythrocyte membranes. A comparison is being made between a acidic phospholipase A2, isolated from a snake venom, and calmodulin. The former has been shown to have properties very close to that of calmodulin.