Arildone (3 micro/ml) reduced the replication of murine cytomegalovirus, Semliki Forest virus, vesicular stomatitis virus, and coxsackievirus A9 by 64, 68, 94, and 98%, respectively. When the plaque reduction method was used to evaluate the antiviral effect for the viruses, a concentration of 3 to 5 micrograms/ml yielded a 50% reduction in plaque numbers. The effect of arildone on virus replication was greatest when the drug was present from the time of inoculation. The effectiveness decreased as the time interval from the inoculation of the virus to the addition of the drug increased. The removal of the drug from infected cells by washing readily reversed the effect, and viral replication resumed at a significant level. Infectivity of these viruses was not inactivated by the drug. Tissue culture cells used for viral growth and assay grew well in arildone (3 micrograms/ml), with cell yields that were comparable to those for cultures in the absence of drug. At 3 micrograms/ml there were minimal effects of the drug on the uptake of 3H-labeled amino acids and [3H]-thymidine into cells. Furthermore, incorporation of these precursors was not affected. However, there was a reduction in uptake of [3H]uridine into the acid-soluble pool and a concomitant reduction in incorporation into acid-insoluble counts.