Under in vitro condition LaCl3 can exhibit a biphasic effect of erythrocyte membrane stabilization and lysis. At a particular concentration of LaCl3 the erythrocytes are protected maximally against hypotonic hemolysis but above or below the optimum level the extent of protection is lowered. At the state of maximum protection there is drastic morphological alteration of the erythrocytes. LaCl3 induced changes of the shape of the erythrocytes are not so marked when the cells are exposed to LaCl3 in an isotonic medium. The delayed onset of hemolysis of chick erythrocytes after in vivo LaCl3 treatment is not associated with morphological alterations of the cells.