BACKGROUND: Human rhinoviruses (HRVs) are responsible for more than half of all cases of the common cold and cause billions of USD annually in medical visits and school and work absenteeism. An assessment was made of the cytotoxic and antiviral activities and possible mode of action of the tannin ellagic acid from the leaves of Lagerstroemia speciosa toward HeLa cells and three rhinoviruses, HRV-2, -3, and -4. METHODS: The antiviral property and mechanism of action of ellagic acid were evaluated using a sulforhodamine B assay and real-time reverse transcription-PCR (RT-PCR) with SYBR Green dye. Results were compared with those of the currently used broad-spectrum antiviral agent, ribavirin. RESULTS: As judged by 50% inhibitory concentration values, natural ellagic acid was 1.8, 2.3, and 2.2 times more toxic toward HRV-2 (38 mug/mL), HRV-3 (31 mug/mL), and HRV-4 (29 mug/mL) than ribavirin, respectively. The inhibition rate of preincubation with 50 mug/mL ellagic acid was 17%, whereas continuous presence of ellagic acid during infection led to a significant increase in the inhibition (70%). Treatment with 50 mug/mL ellagic acid considerably suppressed HRV-4 infection only when added just after the virus inoculation (0 h) (87% inhibition), but not before -1 h or after 1 h or later (<20% inhibition). These findings suggest that ellagic acid does not interact with the HRV-4 particles and may directly interact with the human cells in the early stage of HRV infections to protect the cells from the virus destruction. Furthermore, RT-PCR analysis revealed that 50 mug/mL ellagic acid strongly inhibited the RNA replication of HRV-4 in HeLa cells, suggesting that ellagic acid inhibits virus replication by targeting on cellular molecules, rather than virus molecules. CONCLUSIONS: Global efforts to reduce the level of antibiotics justify further studies on L. speciosa leaf-derived materials containing ellagic acid as potential anti-HRV products or a lead molecule for the prevention or treatment of HRV infection.