Silymarin is a polyphenolic flavonoid that has a strong antioxidant activity and exhibits anti-carcinogenic, anti-inflammatory, and cytoprotective effects. Although its hepatoprotective effect has been well documented, the effect of silymarin on T cells is largely unknown. The purpose of this study was to analyze the effects of the silymarin on the proliferation and cell cycle progression of Jurkat cells, a human peripheral blood leukemia T cell line. Cells were incubated with various concentrations of silymarin for 24-72 h and examined for cell growth and proliferation using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and DNA 5-bromo 2'-deoxyuridine (BrdU) colorimetric assays. Cell cycle analysis by flow cytometry was also performed using propidium iodide staining. Results of the study revealed that silymarin increased proliferation of Jurkat cells at 50-400 microM concentrations with 24 h exposure, confirmed by both MTT and BrdU assays. However, Jurkat incubation with silymarin at higher concentrations of 400 microM for 48 h and 200-400 microM for 72 h caused inhibition of DNA synthesis, cell cycle arrest at the G2/M phase and significant cell death. Results of the present study also revealed a similarity of cell growth patterns between Jurkat, U937 and RPMI 8866 cells. In conclusion, this study demonstrated an in vitro growth stimulatory effect of silymarin on leukemia cells with monocyte, T and B cell origin that has not been previously reported for either solid tumors or other leukemia cells, suggesting a possible specific stimulatory effect of silymarin on the key cells of the immune system.