Four different types of human interferon, interferon-beta (IFN-beta), recombinant IFN-alpha2a and IFN-alpha2b and natural IFN-alpha were tested for antiviral activity against SARS-coronavirus. The experiments were performed using in vitro cultivated monkey Vero E6 cells. IFN-beta was found to be the most highly active antiviral agent, followed by natural IFN-alpha, whereas the 2 recombinant IFN-alpha2 species were poorly active in the system used. These results suggest that IFN-beta as well as natural IFN-alpha may be used for the treatment of SARS.