Gamma-linolenic acid (GLA) is known to have selective tumoricidal action. It is also reported that GLA may increase the cytotoxic activity of cancer chemotherapeutic agents in some cancer cells. The present study examined whether GLA pretreatment could modulate the response of multidrug-resistant K562/ADM leukemic cells to anticancer drugs. The cell viability assay results showed that GLA at 10 microg/ml enhanced cell growth inhibition induced by the MDR-type drugs doxorubicin, etoposide and vincristine, but could not enhance or even attenuated cell growth inhibition induced by the non-MDR-type drug cisplatin, mitomycin and fluorouracil in K562/ADM cells. Further flow cytometry results showed that GLA decreased the expression of P-glycoprotein, enhanced the accumulation of doxorubicin and rhodamine 123 in K562/ADM cells and inhibited the efflux of rhodamine 123 from K562/ADM cells, lowered the efflux rate. These results suggest that GLA could modulate the response to anti-cancer agents in P-gp overexpressing multidrug-resistant cells, and the mechanism may be by decreasing the P-gp expression and inhibiting P-gp function.