Experimental conditions necessary for the full expression of interferon-inducing activity by a complex of polyguanylic acid and polycytidylic acid included: (i) a sufficiently high molecular size of each homopolymer; (ii) annealing conditions which insured complete denaturation of polyguanylic acid self-structure; and (iii) the specific biological assay employed to assay interferon-inducing potency. The complex of polyguanylic acid and polycytidylic acid possessed several properties that suggested it may be an atypical polynucleotide interferon inducer. For instance, it was inactive in primary rabbit kidney cell cultures, usually exquisitely sensitive to polynucleotide interferon inducers, unless it was incubated on the cell cultures for prolonged times or in the presence of DEAE-dextran. Polyguanylic acid X polycytidylic acid could induce interferon in rabbits and mice but gave a more protracted response than did poly(I) X poly(C). Finally, poly(G) X poly(C) was, without any modification, resistant to degradation by serum nucleases.