Antiviral and interferonogenic activity of the complexes of poly(G,A) . poly(C) and poly(G) . poly(C) was studied in mice and cell cultures. Three out of 4 complexes of poly(G,A) . poly(C) had insignificant antiviral and interferonogenic activity in chick embryo cells. One of the complexes induced low levels of interferon production in mice and decreased the rate of their death from experimental forest-spring encephalitis. The activity of poly(G) . poly(C) in the above cell systems was much more pronounced. Unlike this complex, some complexes of poly(G,A) . poly(C) showed a noticeable activity in the cells of Primates. The effect of the noncomplementary base in the purine thread of poly(G) . poly(C) on its biological activity and nucleotide composition is discussed.