The efflux pump P-glycoprotein decreases drug penetration into cells and tissues. To determine whether nelfinavir or its metabolites inhibit P-glycoprotein in lymphocytes from a healthy volunteer, whole blood cells from human immunodeficiency virus-negative donors were incubated either in human plasma to which nelfinavir or its M8 metabolite were added ex vivo or in plasma from human immunodeficiency virus-positive patients receiving nelfinavir. The 50% P-glycoprotein inhibitory concentrations of purified nelfinavir and M8 were 10.9 micromol/L and 29.5 micromol/L, respectively, for CD4(+) T cells and 19.3 micromol/L and >48 micromol/L, respectively, for CD8(+) T cells. Significant inhibitory activity was present in plasma from 27 of 46 patients (59%) receiving nelfinavir. Plasma nelfinavir concentrations correlated with percent inhibition on CD4(+) (rho = 0.85, P <.0001) and CD8(+) (rho = 0.83, P <.0001) T cells. The M8 concentrations correlated weakly with both inhibition and nelfinavir concentrations. On the basis of our findings in lymphocytes from a healthy volunteer exposed to plasma from human immunodeficiency virus-positive patients, we believe it is likely that CD4(+) and CD8(+) lymphocytes in patients receiving nelfinavir as therapy for human immunodeficiency virus may have P-glycoprotein inhibited by plasma concentrations of nelfinavir.