Polyphenol rich extracts obtained from cistus herb (Cistus incanus L.) and pomegranate peels (Punica granatunm L.) added at high concentrations (25-100 pg/mL) to V79 (Chinese hamster lung fibroblast) cell cultures, markedly increased intracellular level of ROS (reactive oxygen species). Also in cells exposed to the exogenous oxidant - H₂0₂, [100 μM, 30 min., 37'C] intracellular ROS level was augmented, although to a lower extent than in resting cells not exposed to H₂0₂. Additionally, cistus and pomegranate extracts at high concentration (25-100 μg/mL) inhibited the P-gp protein efflux function in V79 cell cultures. Both pro-oxidant activity of polyphenols at high concentrations and their ability to decrease efflux function of P-gp (cellular multidrug resistance protein) could be exploited for improving anticancer adjuvant therapy.