The effects of various fatty acid glycerol esters (FAGE) on the transport of ceftibuten in rat intestinal brush-border membrane vesicles (BBMV) were investigated. The HLB numbers of FAGE used were in the range of 6.5 to 15.7. The ceftibuten uptake by BBMV was examined by a rapid filtration method in the presence or absence of FAGE of a non-solubilizing concentration (0.03%, w/v). Tetraglycerol monostearate, hexaglycerol monostearate, hexaglycerol sesquistearate, hexaglycerol tristearate and decaglycerol tristearate were found significantly to enhance the ceftibuten uptake under an inward H+-gradient condition, while FAGE with oleate or laurate did not affect it. The extent of ceftibuten uptake with FAGE correlated with the HLB of the esters (r= -0.90, p<0.001). To elucidate the mechanism of its enhancing action, the effects of FAGE on the membrane lipid fluidity were examined using the fluorescence probes diphenylhexatriene (DPH) and trimethylammonium diphenylhexatriene (TMA-DPH) which distribute in the hydrophobic inner and outer regions of the lipid layer, respectively. The increase in ceftibuten uptake induced by FAGE proved to correlate with an increase in fluidity of the outer lipid layer (r= -0.83, p<0.003) but not the inner lipid layer of BBMV. Hence, the enhancing effect of FAGE with the lower HLB on the ceftibuten transport in the presence of the H+-gradient should, at least in part, be explained by the interaction with the outer lipid regions of BBMV.