Tris acetylacetonate aluminium(III) (Al(acac) 3), dissolved in water, is effective in producing osmotic fragility in suspended erythrocytes in the concentration range of 0.034–0.34 mmol/l. Water solutions of Tris maltolate aluminium(III) (Al(malt) 3) and aluminium lactate (Al(lac) 3) are also effective but the dose-response behavior is less pronounced. Moreover, only Al(acac) 3 induces a prominent generation of acanthocytes. The stronger effects of Al(acac) 3 on membrane stability are attributed to the greater solubility of this complex in the cell membrane.