Epidemiologic evidence indicates that both black and green tea is a rich source of flavonoids and other polyphenolic antioxidants which protects against heart disease and cancer. In the current investigation, utilizing human oral squamous carcinoma cell line SCC-25, we have evaluated the effect of three major tea constituents, (-)-epigallocatechin-3-gallate (EGCG), (-)-epicatechin-3-gallate (ECG) and (-)-epigallocatechin (EGC) on cell growth and DNA synthesis. Test agents in concentrations of 50, 80, 100 and 200 microM were incubated in triplicates in DMEM-HAM's F-12 (50: 50) supplemented with 10% calf serum and antibiotics in an atmosphere of 5% CO2 in air for 72 hrs. Cell growth was determined by alamarBlue assay method and DNA synthesis was measured by the incorporation of [3H]-thymidine in nuclear DNA. At the four dose levels used, the three compounds induced significant dose-dependent inhibition in cell growth. In DNA study, the three compounds exhibited stimulatory effect at 50 microM followed by significant dose-dependent inhibitory effect (10 to 100%) at 80, 100 and 200 microM dose levels. Dose-dependent changes in cell morphology were also observed with phase-contrast microscopy after cell treatment with EGCG.