The cDNA for the rat liver organic anion-transporting polypeptide "oatp" has been shown to encode transport of bromosulfophthalein (BSP) and bile salts in Xenopus oocytes (E. Jacquemin, B. Hagenbuch, B. Stieger, A. W. Wolkoff, and P. J. Meier. Proc. Natl. Acad. Sci. USA 91: 133-137, 1994). Because oatp mRNA is expressed strongly in the kidney, we sought to determine whether renal oatp might play a role in the known secretion of a large variety of organic anions by the kidney. We transiently expressed a full-length oatp cDNA, cloned in pSPORT, in HeLa cell monolayers using the recombinant vaccinia virus vtf7-3. We tested an array of organic anions as candidate substrates by determining their ability to compete with tracer BSP for transport. HeLa cell monolayers transfected with the oatp cDNA transported tracer BSP and taurocholate at rates substantially higher than monolayers transfected with a control plasmid. Thus good expression can be obtained with the vaccinia-HeLa system using a standard plasmid cloning vector. BSP transport varied as a function of the medium albumin, ionic conditions, and pH in a fashion similar to that in Xenopus oocytes. Several organic anions known to be secreted by the classic secretory pathway, including p-aminohippurate (PAH), phenol red, and indigo carmine (10 microM) failed to inhibit oatp-mediated BSP transport. Direct testing using tracers revealed no oatp-mediated transport of sulfate, urate, PAH, several eicosanoids, or unconjugated or conjugated bilirubin. On the other hand, BSP transport was inhibited by approximately 50% by 10 microM corticosterone sulfate, spironolactone, and several other steroids. We conclude that the functional properties of oatp expressed in the HeLa cell/vaccinia transient expression system are comparable to those following expression in Xenopus oocytes and that steroids are likely to represent high-affinity endogenous oatp substrates. The latter hypothesis is addressed in greater detail in a companion paper.