The objective of this study was to determine whether human immunodeficiency virus (HIV) protease inhibitors (saquinavir, ritonavir and nelfinavir) interact with other HIV protease inhibitors and/or HIV reverse transcriptase inhibitory (zidovudine, didanosine, lamivudine, zalcitabine and sanilvudine). We measured transport of nelfinavir, an HIV protease inhibitor which is known as a substrate for the multidrug resistance transporter P-glycoprotein(P-gp), in an epithelial monolayer model and K_i for P-gp of some drugs by a calcein flux assay. Transport in a basal to apical direction was 2-fold greater than apical to basal flux for nelfinavir, K_i for P-gp of a potent P-gp inhibitor cyclosporin A was 1.09 μM and those of ritonavir and nelfinavir were 111 μM and 28.6 μM, whereas all HIV reverse transcriptase inhibitors gave K_i values. These data show that nelfinavir, which is a substrate for P-gp, inhibits a P-gp function as a drug efflux pump and that HIV reverse transcriptase inhibitors do not inhibit P-gp.