Extraction of acidified uremic serum yields an inhibitor of phenytoin binding to normal serum proteins and improves binding to the uremic serum proteins. The extract contains binding inhibitors (Ix) that may accumulate because of poor renal excretion and that are presumed to compete for binding sites on albumin, the principle binding protein in serum. If substantially bound to protein, the normal mode of excretion may be via tubular secretion. In this study, individual extracts from six stable hemodialyzed patients caused significant inhibition of para-aminohippurate (PAH) uptake by rat kidney slices incubated in vitro. During a 90-min incubation, slice-to-medium ratios fell in a curvilinear dose-related pattern to 29.8% +/- (SEM) 3.6% (N = 6) of control values, compared with 56.4% +/- 2.1% after incubation with a similar extract from six normal serum samples. The dose-response curve is similar to that of a known competitive inhibitor, hippuric acid, and the average inhibitory activity of uremic extract as derived from these curves was 3.5 times that of an extract from six normal controls. The effect was easily reversed by washing the slices in medium containing no inhibitor. Whole serum from these same patients had no effect on PAH uptake. These findings suggest that Ix or other constituents of the extract may compete for the organic acid secretory pathway in the proximal nephron. A correlation was seen between inhibition of phenytoin binding to plasma protein and inhibition of PAH transport in the slice. Both factors were only partially removed after prolonged dialysis in vitro, suggesting substantial protein binding. Despite the clinical stability of these hemodialyzed patients, their serum samples contain unidentified inhibitors of ligand binding to plasma proteins and of organic anion transport that share some physiochemical properties.