Incubation of human blood in saline solution of 0-36% (v/v) ethanol for 30 min produces lysis or stabilization of erythrocytes depending on the ethanol concentration. Under less elevated concentrations of ethanol, erythrocytes are present in expanded shapes (R state) that present lower stability and suffer lysis with increase in the ethanol concentration. Under more elevated concentrations of ethanol, erythrocytes are present in contracted shapes (T state) that have higher stability and suffer lysis at even more elevated ethanol concentrations. This work evaluated the effects of glycerol (0 to 2.0 M) and temperature (7 to 47 degrees C) on the stability of the R erythrocytes, characterized by the ethanol concentration at the mid-transition point (D (50R )) of the hemolysis curve (D (50R )). D (50R ) declined sigmoidally with increase in the glycerol concentration or temperature, due to transition of the R to the T state erythrocytes. In 1.5 M glycerol, the erythrocytes stability decreased below 32 but increased above 37 degrees C. The combination of temperature, glycerol and ethanol actions generates a critical value of osmotic pressure below which the R state predominates and above which the T state predominates. At 7 degrees C 1.5 M glycerol decreased the erythrocytes stability against ethanol but increased the erythrocytes stability against hypotonic shock. Those conditions favor the R state, which has a lower stability against ethanol; however, in the absence of ethanol, glycerol determines less water entrance in the erythrocytes, making more difficult its lysis by hypotonicity.