Ribavirin (1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide) at concentrations of 10 microgram/ml or more, inhibited the replication of infectious pancreatic necrosis virus (IPNV) in both Chinook salmon embryo (CHSE-214) and rainbow trout gonad (RTG-2) cells. The drug was most effective when added just before or within 8 h p.i. Incorporation studies with radioactive precursors demonstrated that ribavirin suppressed cellular DNA and RNA synthesis within 2 to 3 h after addition of the drug. The inhibition of nucleic acid synthesis and the antiviral activity was gradually reversed within 3 to 5 days after removal of the drug from the infected cells. Polyacrylamide slab-gel electrophoresis combined with fluorography revealed that: (i) 0.5 microgram/ml actinomycin D sufficiently inhibited host cell RNA synthesis thereby enabling the study of virus-specific RNA synthesis in infected cells and (ii) ribavirin inhibited the synthesis of all three virus RNA forms: the transcription intermediate, virus mRNA and progeny dsRNA.