Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters which play critical roles in the body disposition of clinically important drugs, including anti-HIV therapeutics, antitumour drugs, antibiotics, anti-hypertensives and anti-inflammatories. hOAT4-mediated transport of the organic anion oestrone sulphate in COS-7 cells was inhibited by the histidine-modifying reagent DEPC (diethyl pyrocarbonate). Therefore the role of histidine residues in the function of hOAT4 was examined by site-directed mutagenesis. All five histidine residues of hOAT4 were converted into alanine, singly or in combination. Single replacement of His-47, or simultaneous replacement of His-47/52/83 or His-47/52/83/305/469 (H-less) led to a 50-80% decrease in transport activity. The decreased transport activity of these mutants was correlated with a decreased amount of cell-surface expression, although the total cell expression of these mutants was similar to that of wild-type hOAT4. These results suggest that mutation at positions 47, 47/52/83 and 47/52/83/305/469 impaired membrane expression rather than function. We also showed that, although most of the histidine mutants of hOAT4 were sensitive to inhibition by DEPC, H469A (His-469-->Ala) was completely insensitive to inhibition by this reagent. Therefore modification of His-469 is responsible for the inhibition of hOAT4 by DEPC.