Certain uremic metabolites are recognized to have high affinity to serum protein and some of them have been identified as hippuric acid (HA), quinolinic acid (QA) and indoxyl sulfate (IS). Cell toxicity and effect on renal function of these substances were examined by means of erythroid colony formation, lymphocyte blast formation and isolated perfused rat kidney. These substances inhibited the binding of diphenylhydantoin to albumin, depending on the concentration of the substances. At the same concentration of 10 mg/dl, IS was most potent and QA was the second. QA and IS suppressed the erythroid colony formation, depending on the concentration of QA and IS. On the other hand, HA had no suppressive effect even at the higher concentration. The suppressive effect of QA and IS were attenuated by increasing erythropoietin concentration. QA and IS had strong suppressive effect of lymphocyte blast formation and interleukin 2 production at the concentration of uremic serum. However, they did not suppress the increase of intracellular calcium concentration of lymphocytes after stimulation by mitogen. This might indicate the possibility that these substances act not only on cell surface but on intracellular protein. HA and IS inhibited para-aminohippurate secretion and QA suppressed tubular reabsorption of sodium in isolated perfused rat kidney. These results show that the uremic protein binding inhibitors may influence renal regulation of fluid and electrolyte homeostasis. It is concluded that some of the protein binding inhibitors have toxic effects on cell function of various tissues and play a role in pathophysiology of uremia.