Erythrocytes from different vertebrate classes were tested for susceptibility towards copper ion-induced lysis under identical copper ion concentration and per cent cell volumes. 2. The susceptibility towards lysis was found to be correlated with the rate of copper ion entry into the erythrocytes. 3. GSH levels decline in red blood cells at a rate proportional to the rate of copper ion entry. 4. Hemolysis does not seem to be causally related to the level of GSH in the erythrocytes.