Halogenated nucleosides such as 5′-fluoro and 5′ iodo-2-′deoxyuridine have been useful in studying the kinetics of replication of certain DNA containing viruses (Shatkin & Salzman, 1963) and also in the chemoprophylaxis of certain infections caused by these viruses (Jawetz, Coleman, Dawson & Thygeson, 1970). Riboviruses with exception of oncornaviruses (Fenner, 1968) are unaffected by concentrations of FUDR which completely inhibit the multiplication of deoxyriboviruses. However, certain nucleoside analogues inhibit the replication of RNA containing viruses. The adenosine analogue cordycepin inhibits ribosomal, transfer and cytoplasmic messenger RNA perhaps by direct inhibition of the DNA dependent RNA polymerase reaction by incorporation into the 3′ terminal position of nascent RNA (Plagemann, 1971) and causes 80% inhibition of NDV multiplication (Mahy, Cox, Armstrong & Barry, 1973). More recently a synthetic nucleoside analogue l-β-D-ribofuranosyl-l,2,4-triazole- 3-carboxamide (ribavirin or RTCA) has been shown to inhibit the multiplication of both DNA and RNA-containing viruses (Sidwell et al., 1972; Streeter et al., 1973). We describe here the inhibition of influenza type A and B virus replication in tissue culture cells by RTCA and show that the compound acts at an early stage in the influenza-infected cell, possibly by inhibiting the production of essential nucleotides and hence RNA synthesis, although an action of the compound via an inhibitory effect on cell DNA synthesis is not excluded.