The chemopreventive effect of polyphenols from green tea [e.g. (-)-epigallocatechin gallate (EGCG) and (-)-epigallocatechin (EGC)] against cancer has been demonstrated in several studies. The aim of this investigation was to prove whether these compounds modulate the activity of antineoplastic drugs. Therefore, the influence of EGCG and EGC was tested on doxorubicin-resistant murine sarcoma (S180-dox) and human colon carcinoma (SW620-dox) cell lines. Both substances showed a sensitizing effect on the cell lines if they had been treated with doxorubicin. These results suggest that protein kinase C may be inhibited by EGCG and EGC, and this may lead to a reduced expression of some drug resistance related proteins.