Various extracts from the leaves of mistletoe (Viscum album L. ssp. album) were investigated for their antiviral activity on human parainfluenza virus type 2 (HPIV-2) growth in Vero cells. Plant extracts were prepared using distilled water, 50% ethanol, petroleum ether, chloroform and acetone. The 50% effective dose (ED50) of aqueous extract for HPIV-2 replication was 0.53 +/- 0.12 mug/mL, and the antiviral index (AI), which was based on the ratio of the 50% inhibitory concentration (CD50) for host cell viability to the ED50 for parainfluenza virus replication, was 10.05. The aqueous extract was found to be the most selective inhibitor. Furthermore, the aqueous extract at a concentration of 1 mug/mL was found to inhibit HPIV-2 replication and the virus production was suppressed to more than 99% without any toxic effect on host cells. The chloroform extract was also found to be moderately active. In an effort to further analyse the mechanism of antiviral activity, the effectiveness of the aqueous extract on different steps of virus replication was examined. The antiviral activity could neither be attributed to the direct inactivation of the HPIV-2 nor to the inhibition of adsorption to Vero cells. The active aqueous extract has shown a dose-dependent antiviral activity on virus replication. Copyright (C) 2003 John Wiley Sons, Ltd.