Exposure of rabbit or human erythrocytes to concentrations of puromycin as low as 7 x 10(-4)M for 2 hr causes damage to the cell membrane, as evidenced by increased susceptibility of the cells to hyposmotic lysis, increased cell rigidity, and ultrastructural changes consistent with severe membrane damage. Puromycin causes a concentration-dependent internalization of the erythrocyte membrane, resulting in vacuolization of the cells, at concentrations between 7 x 10(-4) M and 10(-2) M. Since the erythrocyte does not synthesize protein, the data indicate that puromycin has a direct toxic effect on erythroid cell membranes which is unrelated to its action in inhibiting the synthesis of protein.