The antiviral activity and conditions of formation of the most active double-stranded complexes of synthetic homopolynucleotides, polyriboguanylic and polyribocytidylic acids, were studied on the model of primary trypsinized chick embryo cells and RNA-containing viruses. The (poly G).(poly C) complex was very active against the viruses tested; their replication in cell cultures was inhibited completely. The antiviral activity of the (poly G).(poly C) complex increased markedly in the presence of diethylaminoethyl- (DEAE-) dextran. After treatment with 1 mug/ml of (poly G). (poly C) for 1 hour in the presence of 100 mug/ml DEAE-dextran, the cell sheet remained protected for 5-7 days. Preparations of (poly G).(poly C) obtained under optimal conditions were as active as (poly I).(poly C) complexes and exceeded them markedly in the level of the therapeutic index which under the present experimental conditions was 5-10 times 10(3) for (poly G).(poly C). Highly purified homopolymers with sufficiently high molecular weight must be used for production of active and stable (poly G).(poly C) complexes.