Five distinct organic anion transporter cDNAs, hOAT1-5, were isolated from human liver and kidney. hOAT1, 2, and 3 are homologous to their respective rat orthologues OAT1-3, whereas hOAT4 and 5 are novel clones that have not been identified in other species. hOAT1- and hOAT3-transfected cells showed uptake of p-aminohippurate and fluorescein. Cells expressing hOAT2 showed uptake of p-aminohippurate, methotrexate, cAMP, and alpha-ketoglutarate. Northern blot analysis indicated differential tissue distribution for the transporter transcripts. These results indicate the existence of a family of organic anion transporting proteins in humans distinct from the oatp-like family of transporters.