Ten strains of varicella-zoster virus (VZV) were tested for susceptibility to 17 nucleoside analogues by a plaque reduction assay using human embryonic lung fibroblast cells. The compounds employed were 5-substituted arabinosyluracils and 2'-deoxyuridines, 2'-fluoro-arabinosylpyrimidines (F-araPyr) and acyclovir. In terms of the 50% plaque reduction dose (PD50), 4- to 40-fold difference were found between the 10 strains of VZV in susceptibilities to each compound. VZV was highly susceptible to 5-halogenovinyl-arabinosyluracils (XV-araUs); the PD50 values of these compounds were less than 0.001 micrograms/ml. VZV was much more susceptible than herpes simplex virus (HSV) type 1 to XV-araUs, but less susceptible than either HSV type 1 or type 2 to 5-ethyl-2'-deoxyuridine, 5-ethyl-arabinosyluracil and acyclovir.