The antiviral and antimetabolic activities of poly(7-deazaadenylic acid) [poly(c⁷A)]¹, poly(7-deazainosinic acid) [poly(c⁷I)] and their nucleosides 7-deazaadenosine (c⁷A) and 7-deazainosine (c⁷I) have been assessed in primary rabbit kidney (PRK) and mouse JLSV5 cells, the latter being a transformed, (Rauscher) murine leukemia virus-shedding cell line. The antiviral activity was monitored by investigating the effects of the compounds on the replication of vaccinia, herpes simplex or vesicular stomatitis virus in PRK or JLSV5 cells and by measuring endogenous oncornavirus production in JLSV5 cells. The inhibition of cellular RNA and DNA synthesis served as parameters of antimetabolic activity. With all compounds tested the minimum effective concentrations inhibiting virus replication coincided remarkably well with the minimum toxic concentrations impairing cellular RNA and DNA synthesis or normal cell morphology. These concentrations amounted to 0.3–0.6 μg/ml for c⁷A and poly(c⁷A), 3 μg/ml for c⁷I and 10 μg/ml for poly(c⁷I). At higher concentrations (～-70 μg/ml) poly(c⁷A) and poly(c⁷I) were also found to inhibit the in vitro DNA polymerase activity of (Moloney) murine leukemia virus. According to the data presented, poly(c⁷I), poly(c⁷I) and the corresponding monomers c⁷a and c⁷I cannot be considered as specific antiviral or antitumor agents.