The cell membrane, in addition to other functions, plays an important role in regulating cell metabolism governed by messenger proteins (or other substances) acting from the outside. The interferon receptor system located in the cell membrane (used as a model) might consist of two components: a binding site and an activator site. As shown by experiments based on competition between interferons for the same receptor, binding is not necessarily followed by activation of the antiviral state. It is possible that polysaccharid residues present in gangliosides play an important role in binding. A critical concentration of interferon molecules in contact with the receptors is needed to induce the antiviral state, which is thus a cooperative process. The activation and probably the amplification of the response require free membrane-bound energy and the integrity of the cytoskeletal components of the cell. Modifications in cell membrane structure can change the response to interferon; on the other hand, interferon might induce changes in the cell membrane which finally result in an altered response to toxins and, in some instances, in recovery of lost contact inhibition in transformed cells.