The effect of newly synthesized halogenated isoflavans and isoflavenes on human rhinovirus 1B (HRV 1B) infection of HeLa cells has been examined. Both series of drugs inhibited virus plaque formation in cell cultures, isoflavans being more effective than isoflavenes. Cells pretreated with compounds before challenge with HRV 1B became resistant to the virus-induced cytopathic effect. The antiviral state induced by the most active compounds persisted for at least 10 h and did not appear to be mediated by interferon production. Experiments whereby the compounds were added at varying times indicated that the isoflavans and isoflavenes interfere with early events of virus replication without affecting virus binding to the cell membrane. In addition to their effects on virus multiplication, the isoflavans were also found to have a direct action on the virus. The inhibitory effect on virus infectivity required extraction with chloroform for reversal. Isoflavans also protected the virions against mild acid or heat inactivation.