The intracellular and plasma concentrations of HIV protease inhibitors (HPIs) vary widely in vivo. It is unclear whether there is a concentration-dependent effect of HPIs such that at increasing concentration they may either block their own efflux (leading to 'autoboosting') or influx (leading to saturability/decreased intracellular accumulation). The effects of various concentrations (0-30 mu M) of lopinavir, saquinavir, ritonavir and atazanavir on the accumulation of [C-14]lopinavir, [H-3]saquinavir, [H-3]ritonavir and [H-3]atazanavir, respectively, were investigated in CEMparental, CEMVBL [P-glycoprotein (ABCB1) overexpressing], CEME1000 (MRP1 overexpressing) and in peripheral blood mononuclear cells (PBMCs). We also investigated the effects of inhibitors of ABCB1/ABCG2 (tariquidar), ABCC (MK571) and ABCC1/2 (frusemide), singly and in combination with HPIs, on cellular accumulation. In all the cell lines, with increasing concentration of lopinavir, saquinavir and ritonavir, there was a significant increase in the cellular accumulation of [C-14]lopinavir, [H-3]saquinavir and [H-3]ritonavir. Tariquidar, MK571 and frusemide (alone and in combination with lopinavir, saquinavir and ritonavir) significantly increased the accumulation of [C-14]lopinavir, [H-3]saquinavir and [H-3]ritonavir. Ritonavir (alone or in combination with tariquidar) decreased the intracellular accumulation of [H-3]ritonavir in PBMCs. Atazanavir decreased the accumulation of [H-3]atazanavir in a concentration-dependent manner in all of the cells tested. There are complex and variable drug-specific rather than class-specific effects of the HPIs on their own accumulation.