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Immunosuppression is a life-threatening complication of late cancer stages. In this regard, overproduction in the host plasma of the anti-inflammatory cyclopentenone prostaglandins (CP-PGs), which are strongly antiproliferative at high concentrations, may impair immune function. In fact, lymphoid tissues of tumour-bearing rats accumulated large amounts of CP-PGs while the tumour tissue itself did not. Expression of the CP-PG-induced 72-kDa heat shock protein (hsp70) was elevated in lymphocytes from tumour-bearing animals related to controls. As the capacity for CP-PG uptake by lymphocytes is the same as tumour cells, we investigated whether the latter could overexpress the multidrug resistance-associated protein (MRP1/GS-X pump) which extrudes CP-PGs towards the extracellular space as glutathione S-conjugates. Walker 256 tumour cells extruded 15-fold more S-conjugates than lymphocytes from the same rats (p < 0.001). This did not appear to be related to deficiency in lymphocyte glutathione (GSH) metabolism, since the major GSH metabolic routes are consistent with CP-PG conjugation in lymphocytes. This was not the case, however, for the MRP1/GS-X pump activity in lymphocyte membranes (in pmol/min/mg protein: 3.1 +/- 1.7 from normal rats, 0.2 +/- 0.2 from tumour-bearing animals vs 64.3 +/- 7.0 in tumour cells) which was confirmed by Western blot analysis for MRP1 protein. Transfection of lymphocytes with MRP1 gene completely abolished CP-PG (0-40 microM) toxicity. Taken together, these findings suggest that CP-PG accumulation in lymphocytes may be, at least partially, responsible for cancer immunodeficiency. Clinical approaches for overexpressing MRP1/GS-X pump in lymphocytes could then play a role as a tool for the management of cancer therapeutics., (Copyright 2005 John Wiley & Sons, Ltd.)