Cells exposed to the complex of polyinosinic and polycytidylic acid (poly IC) develop resistance to direct virus challenge and simultaneously release interferon (Field et al. 1967; Younger & Hallum, 1968). The mechanisms by which poly IC triggers the production of interferon are unknown. The effect of the time of exposure of cells to poly IC on interferon induction was examined in order to clarify the nature of ‘triggering’. Since all the antivirus activity of poly IC is not necessarily due to the induction of interferon (Kjeldsberg & Flikki, 1971), we determined simultaneously the yield of interferon and the intracellular resistance to virus as functions of the poly IC concentration and the time of exposure. Our results indicate that the critical adsorption of molecules of poly IC by human cells is much more rapid than previously suspected.