THE ERA OF ANTIVIRALS Introduction Although there are more than one hundred medically useful antibiotics and fungicides, there are only seven compounds licensed for use as antiviral agents, in the USA. Some of these (acyclovir and ganciclovir) are actually derivatives of each other, making the number of new discoveries even smaller. Moreover, most of these agents are of only limited therapeutic value and have substantial toxicity. It has been more than 100 years ago since Pasteur studied rabies virus (2) and Rous (4) showed that a small filterable agent (not bacteria) caused disease (sarcoma) in chickens. It was nearly 100 years ago that yellow fever virus, the first recognized human pathogenic virus, was unambiguously associated with disease (3). Enteroviruses were cultured for the first time nearly 50 years ago (1). Why then has effective chemotherapy against viruses lagged behind that of other microorganisms? Viruses are often difficult to grow and image. However, with the dynamic advances in molecular biology and increased sophistication in tissue culture, the field of virology has blossomed and resulted in improved methods for detection of virus infection. The use of viruses as models of gene regulation and replication has also resulted in a massive accumulation of information.