The studies chronicled in this chapter were chosen largely because they contained data amenable to the quantitative analysis of interferon induction dose-response curves and the IFP activity they represented. The interpretation of the data was predicated on the assumption that a single molecule of dsRNA, when properly introduced into a cell, either as a preformed entity or formed therein following some synthetic event, can induce a quantum yield of interferon. This novel view of interferon induction by viruses has provided an explanation for many seemingly discordant results and offers a unifying hypothesis regarding the nature of the interferon inducer moiety for viruses from widely different families. If we note the reluctance of some to accept dsRNA as a common interferon inducer molecule ( McKimm and Rapp , 1977; Kowal and Youngner , 1978; Joklik , 1980) but recognize that the threshold for activating the interferon induction system is one molecule per cell, many aspects of interferon induction heretofore enigmatic are rendered offerpretable . Furthermore, one molecule of dsRNA per cell suffices to induce a quantum yield of interferon, an apparent expression of the "one-shot affair" of interferon production recognized by Ho (1964). In some cases of induction (the r = 1 type dose-response curve) there is an exquisitely responsive modulation of production when a second molecule of dsRNA is simultaneously introduced into the cell. The experimental approach and concepts discussed herein offer a new perspective on the mechanism of interferon induction by viruses and its regulation, and point out the incredible biological potency of a dsRNA molecule--a molecule found to play a key role in viral infection and host defence (Carter and De Clercq , 1974), regulation of the immune system (Johnson, 1980), and perhaps some yet to be defined function in cell growth (Taylor- Papadimitriou , 1980) and differentiation ( Grossberg and Sabran , 1981-2) through its capacity to activate the interferon system.