The effects of low levels of glutaraldehyde uptake (less than 120 mumol/10(10) cells) on the physicochemical properties of human red blood cells (RBC) were investigated. Salient effects include: by different measures of cell deformability, the extent of glutaraldehyde uptake required to decrease cellular deformability was shown to range from approximately 8 to 30 mumol/10(10) cells; osmotically stressed red cells exhibit complete hemolysis when the level of glutaraldehyde uptake is less than 28 mumol/10(10) cells and no hemolysis when uptake is less than 70 mumol/10(10) cells with the extent of hemolysis decreasing in an approximately linear manner with glutaraldehyde uptake between these limits; glutaraldehyde uptake of up to 58 mumol/10(10) cells does not change the cells' density, mean cell volume or ability to retain potassium.