The pyrimidine nucleoside analog FIAC and its primary deaminated uracil metabolite FIAU are highly and specifically active compounds in vitro against several herpes group viruses, particularly herpes simplex virus (HSV) types 1 and 2, varicella zoster (VZV), and cytomegalovirus (CMV), as well as hepatitis B virus (HBV). Since FIAU is the primary metabolite of FIAC and the administration of FIAU simplifies the metabolism of FIAC, it is anticipated from clinical studies of FIAC that FIAU will be tolerated at least as well as FIAC. A single-dose, pharmacokinetic (blood level) study showed that FIAC, when taken orally, is readily absorbed into the bloodstream, and most of it is converted to FIAU. Daily oral doses are expected to provide concentrations of FIAU exceeding the in vitro minimum inhibitory concentration for nearly all the herpes group viruses.