In cultures of mouse or chick embryo ceils several non-steroidal antiinflammatory drugs (salicylates, Ibufenac, phenylbutazone, Indomethacin, flufenamic and mefenamic acids, CI-583, and chloroquine) inhibit the multiplication of various viruses (encephalomyocarditis, Sindbis, influenza A2, Newcastle disease, herpes simplex and vaccinia). Two factors were critical for the virus inhibitory action...