The mechanism by which proteins that pass through the glomerular basal lamina are taken up by proximal tubule cells is incompletely characterized. Past work has identified the kinetics of albumin binding to renal brush-border membrane. We have now purified and characterized albumin binding protein (ABP) and shown its distribution in renal proximal tubular cells. ABP was purified from rat renal proximal tubular cell brush-border membrane by affinity chromatography with rat serum albumin-Sepharose. The resulting ABP had two apparent molecular masses (55 and 31 kDa) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Antibodies to ABP were raised in rabbits and checked by immunoassay and immunoblotting. Light-microscopic immunohistochemistry showed ABP all along the proximal tubule in the pars convoluta and pars recta. Electron-microscopic immunohistochemistry showed labeling on microvilli and in apical endocytic vacuoles, dense apical tubules, and lysosomes. These results indicate that ABP is involved in proximal tubule endocytosis.