Green tea (Camelia sinensis) is known to possess biological properties that are antioxidative and antimutagenic. Recent studies demonstrated beneficial effects of green tea in inflammatory allergy. However, the effect of green tea on anti-allergic activity/IgE responses in vitro has not been studied. U266 myeloma cells (2 x 10(6)/ml), which secrete IgE, were cultured for 0-72 hr with or without green tea extract (1-300 ng/ml), and IgE levels in the supernatants were determined (24-72 hr) by ELISA. The effects of green tea extract on U266 cell numbers, viability, and apoptosis were studied by flow cytometry. High levels of IgE produced by U266 cells were observed at 24, 48, and 72 hr (1.3 +/- 0.3 x 10(3), 1.7 +/- 0.3 x 10(3), 2.8 +/- 0.4 x 10(3) IU/ml, respectively). Addition of green tea extract either as (a) a single dose, or (b) repeated daily doses, suppressed IgE production with increasing suppression over time (up to 90%; p <0.05); the suppression was dose-dependent with the highest concentrations resulting in the greatest suppression. The suppression of IgE production by green tea extract was not mediated by apoptosis or cell death. This study demonstrates that green tea extract has immunoregulatory effects on human IgE responses in vitro.