Tetrandrine is a new calmodulin antagonist.It can specifically inhibite CaM-stimulated Ca2+-Mg2+-ATPase in human erythrocyte membrane.At higher concentration of Tet,it also inhibites the activity of basal-Ca2+-Mg2+-ATPase,Na+-K+-ATPase and Mg2+-ATPase at the different degree.In addition to CaM,unsaturated fatty acids and limited proteolysis with trypsin can also activate the Ca2+-Mg2+-ATPase.All these activation are inhibited by Tet in a similar cncentra-tion range.This fact suggests that in addition to CaM Tet may bind to membrane Ca2+-Mg2+-ATPase.Addition of Tet may also provide some protection against osmotic lysis of red blood cells.This function reflects that the inhibition of CaM function may be related to drug-induced membrane stabilization.