Recently, we isolated the multispecific organic anion transporter (OAT1) from the rat kidney, which plays important roles in the renal elimination of endogenous and exogenous organic anions including clinically important drugs. In the present study, we cloned and characterized human OAT1. Two cDNA clones, hOAT1-1 cDNA and hOAT1-2 cDNA, were isolated from a human kidney cDNA library, whose amino acid sequences were 86.0% and 87.8% identical to that of rat OAT1, respectively. When expressed in Xenopus laevis oocytes, hOAT1 mediated sodium-independent uptake of p-aminohippurate (PAH) (Km = 9. 3 +/- 1.0 microM). hOAT1-mediated PAH uptake was inhibited by bulky inorganic anions, various xenobiotics, and endogenous substances, including benzylpenicillin, furosemide, indomethacin, probenecid, phenol red, urate, and alpha-ketoglutarate. Northern blot analysis revealed that hOAT1 mRNA is strongly expressed in human kidney; transcripts of different sizes are expressed in skeletal muscle, brain, and placenta. Immunohistochemical analysis using rabbit IgG antibody against the carboxy-terminal 14 peptides of hOAT1 revealed that hOAT1 is expressed at the basolateral membrane of the proximal tubule. hOAT1 gene was located on human chromosome 11q13.1 by fluorescent in situ hybridization analysis. These results indicate that hOAT1 is a multispecific organic anion transporter on the basolateral membrane of the proximal tubule in human kidney.