MANY animal cell types exhibit the ability to communicate between themselves, both in vivo and in vitro1. This phenomenon is thought to occur through gap junctions which allow cells to share their metabolites and small control molecules2-6. The overall effect of this cellular communication seems to be a coordinated control of growth and function of different cells and cell types. Interferon is believed to act at the cell membrane in a fashion similar to polypeptide hormones7-9. This membrane interaction in turn leads to de-repression and production of the antiviral protein10,11. If, as is the case with polypeptide hormones12, the induction of the antiviral protein is mediated through secondary molecules, these might influence adjacent cells. The presence of such intermediary molecules during interferon induction of the antiviral protein has been inferred from kinetic studies13,14. The many instances of the species specificity of interferon action15 make this a testable hypothesis. We show here that cells made resistant to virus infection by treatment with their homologous interferon can transfer viral resistance to cells of a heterologous species insensitive to that interferon and also to cells of the homologous species. © 1977 Nature Publishing Group.