Sephadex gel filtration of the 1000,000 g supernate of homogenates of rat kidney revealed binding of various organic anions (penicillin, Bromsulphalein [BSP], bilirubin, phenolsulfonphthalein [PSP], phlorizin, glutathione [GSH], p-amino hippurate (PAH), probenecid, conjugated bilirubin, and BSP-GSH) to a nonalbumin-containing protein fraction (Y), which precipated on addition of monospecific anti-rat liver ligandin (Y protein)-IgG, but not control IgG. Quantitatively similar organic anion binding was observed in vivo after injection of BSP, BSP-GSH, phlorizin, probenecid, conjugated bilirubin, PAH, or penicillin. The binding protein was purified to apparent homogeneity and is a basic protein (pI 8.9) of 44,000 daltons with two apparently identical subunits of 22,000 daltons. Monospecific antibody was produced against the renal protein. The results of binding studies in vivo and in vitro and phsicochemical, immunologic, structural, and binding site investigations indicate that the renal protein is identical to hepatic ligandin. Immunofluorescent studies utilizing anti-ligandin IgG previously localized ligandin in the kidney to all proximal tubular cells. By quantitative radial immunodiffusion, the concentration of renal ligandin was 31.2 plus or minus 2.2 mug/mg supernatant protein and was increased 160% above basal values by pretreatment of rats with tetrachloro-dibenzo-p-dioxin. Pretreatment with phenobarbital, DDT, or pregnene-16alpha-carbonitrile did not increase renal ligandin concentration but doubled hepatic ligandin concentration. Circular dichroism studies of renal ligandin revealed percent helical structure similar to hepatic ligandin and primary association contrasts were derived for BSP (10-6 M-1) and PAH, probenecid, and penicillin (10-3 M-1). Administration of BSP or probenecid simultaneously with [C14] penicillin resulted in increased plasma retention and reduced kidney and urinary bladder content of [14C] penicillin and a correlation coefficient of -0.8 between total kidney/plasma radioactivity and percent of protein-bound radioactivity bound to ligandin in the kidney. These studies indicate that renal and hepatic ligandin are identical. Their response to drugs and chemicals varies. Competitive binding between several organic anions for ligandin correlated with their renal uptake from plasma, which suggests that ligandin may function in the proximal tubular cell as a component of the renal organic anion transport system.