A large variety of displacement reactions between homopolynucleotides and complexes thereof has been demonstrated by interferon induction data obtained in primary rabbit kidney cell cultures superinduced with metabolic inhibitors. The polymers involved in these helix-coil displacement studies were: poly(adenylic acid), poly(inosinic acid), poly(cytidylic acid), poly(uridylic acid), poly(ribothymidylic acid), polylaurusin, poly(7-deazaadenylic acid), poly(7-deazainosinic acid), poly(5-bromocytidylic acid), and poly(5-bromouridylic acid). As monitored by ultraviolet absorbance-temperature profiles, all displacement reactions were directed toward the formation of the helix with the higher thermal stability. Concomitantly, the resulting helix was invariably more active as interferon inducer than the reactant helix, except for some reactions in which poly(7-deazaadenylic acid) was involved. For the latter reactions both the reactant and resultant helices were inactive as interferon inducer. The interferon induction data revealed that all displacement reactions proceeded to completion within 1 h even at temperatures well below the Tm of the reactant helix. The helix-coil displacement reaction could also be monitored by sucrose velocity gradient analysis, and, as evidenced for poly(A)-2poly(I) + 2poly(C) leads to 2poly(I)-poly(C) + poly(A), readily occurred at the cellular level, presumably at the cell surface.