Pentoxifylline (I) [3,7-dimethyl-1-(5-oxo-hexyl)-xanthine] (0-10(-3) M) inhibited the discocyte echinocyte transformation caused by hyperosmolarity and improved the impaired filtrability of erythrocytes due to hyperosmolarity. 2. The ATP content of erythrocytes was increased at low concentrations (0-3 x 10(-5) M) of I, but decreased at high concentrations (3 x 10(-5) M or above) of I.