In summary we have shown that interferon treated mouse L cells can transfer their antiviral resistance to cocultured heterologous (human WISH or hamster BHK) cells which are insensitive to mouse interferon. Transfer of viral resistance seems to be initiated by interferon itself. Once transferred the viral resistance has the characteristics of the interferon-induced antiviral state. The transferred resistance occurs between several cell species. The transfer of resistance depends on the ratio of cells homologous (to interferon) to cells which are heterologous as well as to the absolute cell density at a given ratio. The transfer process is efficient in that it requires relatively small amounts of interferon. Finally, we propose that this phenomenon is a natural process for amplification of the interferon system and preliminary evidence indicates that it occurs by cell to cell transfer of an interferon-induced molecule.