Effective antiviral agents are needed to treat severe acute respiratory syndrome-associated coronavirus (SARS-CoV) infection. We assessed the antiviral potential of recombinant interferons against two clinical isolates of SARS-CoV--FFM-1, from Frankfurt patients, and Hong Kong--replicated in Vero and Caco2 cells. Interferon beta was five to ten times more effective in Caco2 cells. Interferon alpha effectively inhibited SARS-CoV replication, but with a selectivity index 50-90 times lower than that for interferon beta. Interferon gamma was slightly better than interferon alpha in Vero cell cultures, but was completely ineffective in Caco2 cell cultures. Interferon beta could be useful alone or in combination with other antiviral drugs for the treatment of SARS.