3′-Deoxyadenosine, 3′-amino-3′-deoxyadenosine (3′3′), nine derivatives therefrom, two derivatives of 5′-amino-5′-deoxyadenosine and three derivatives of 3′, 5′-dideoxyadenosine were tested in cell culture for antiviral activity against three DNA viruses: adenovirus 5, herpesvirus hominis 1, and vaccinia virus. Cytotoxicity was also assessed. (3′3′) derivatives affected the multiplication of all three viruses similarly. Those which were effective were also cytotoxic at the same or slightly higher concentration. Substitution or other molecular modification of (3′3′) tend to lower the biological activity. The presence of an adenosine deaminase inhibitor enhanced both antiviral activity against adenovirus 5 and cytotoxicity in (3′3′) compounds, but not in the others. Both 5′-amino-5′deoxyadenosine compounds were active against vaccinia virus only. Of the three 3′, 5′-dideoxyadenosines, only one had both cytotoxic and antiviral activity. Most, if not all, seemingly antiviral effects appear to be caused