Phytohaemagglutinin (PHA M and P forms), when added to L 929 cells previously treated by murine interferon, degrades the antiviral state and restores virus sensitivity in the cells. This degradation depends on the amount of the lectin, the contact period with the cell and the presence of PHA membrane receptors. The importance of membrane configuration not only in the induction but also in the maintenance of the antiviral state is discussed.