The development of pyrimidine and purine nucleoside derivatives and the demonstration of their in vitro and in vivo antiviral effects have been among the major achievements of viral chemotherapy during the past decade. Nevertheless, despite increasing use of such agents to treat disseminated varicella–zoster, herpes-simplex encephalitis, herpes-simplex neonatorum and cytomegalic inclusion disease there is no convincing evidence of their clinical efficacy.The possibilities and limitations in the development of antiviral agents become clearer if several features of virus–cell interaction are understood. In the first place, viruses are unique parcels of genetic material that as obligate intracellular parasites are dependent on . . .