A defined, decaffeinated green tea polyphenol mixture isolated from Camellia sinensis, a plant native to Asia, with antiviral and antioxidant activities and potential chemopreventive activity. Green tea extract contains antioxidant compounds, including flavonoids, vitamins and polyphenols such as epigallocatechin-3-gallate (EGCG), which may have antineoplastic properties. Consumption of green tea extract may confer chemopreventive protection against various cancers including those of the prostate, stomach, and esophagus.