Synthetic polyribonucleotides stimulate cells to produce interferons and other cytokines and increase both humoral and cell-mediated immunity. The polynucleotide complexes affect host defense system and may have antiviral properties. Studies in animal models of experimental viral infection may therefore be performed to define a possible strategy for their use in human disease. Antiviral properties of polyribonucleotides have been demonstrated in animals, especially in murine models of retroviral infection. In the early treatment of Friend virus infection, the antiretroviral effect of zidovudine is enhanced by combination with poly I poly C or poly A poly U. Polyribonucleotides also enhance the inhibitory effect of zidovudine on HIV replication in lymphocyte T or macrophage cultures. Therefore this class of compounds could be used in combination with antiviral agents in the treatment of HIV infection, especially when they induce no significant toxicity as it is the case for poly A poly U.