2-Amino-9H-purine in which the hydrogen at position 9 is substituted by a 4-acetoxy-3-(acetoxymethyl)but-1-yl group. A prodrug of the antiviral penciclovir, it is used for the treatment of acute herpes zoster (shingles), for the treatment or suppression of recurrent genital herpes in immunocompetent patients and for the treatment of recurrent mucocutaneous herpes simplex infections in HIV infected patients.