Cicloxolone is a broad spectrum antiviral agent with a largely non-specific and complex mode of antiviral action. The drug was active during all stages of the virus replication cycle, indicating that it does not operate by the specific inhibition of any single essential virus gene product. The drug reduced the number of vesicular stomatitis virusparticles assembled and released by 100- to 1000-fold. Infectious virus yield was reduced 1000- to 10000-fold, giving a 10-fold or greater increase in the particle/p.f.u. ratio. The reduced number of virus particles produced in the presence of Cicloxolone results from two superimposed effects: suppression of vesicular stomatitis virussecondary transcription and viral protein synthesis, and perturbation of virion assembly.