N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP) is a physiological inhibitor of hematopoiesis that is maintained at stable levels in normal plasma. Its degradation in vivo and in vitro by angiotensin-converting enzyme (ACE) accounts for the high plasma concentrations of AcSDKP in patients treated with ACE inhibitors. Because ACE inhibitors can induce anemia in some patients, we measured plasma AcSDKP concentrations in 176 patients with chronic renal failure: 120 hemodialysis (HD) and 56 nondialysis (nD) patients, 39 of whom were administered ACE inhibitors. We studied the relationships between AcSDKP levels, hematologic parameters, and recombinant human erythropoietin (rHuEPO) requirements in these patients. AcSDKP levels were significantly greater in HD (10.3 +/- 3.9 pmol/mL) and nD (3.1 +/- 1.8 pmol/mL) patients not administered ACE inhibitors than controls (1.8 +/- 0.2 pmol/mL). In all patients, treatment with ACE inhibitors significantly increased these levels fourfold. HD sessions significantly decreased AcSDKP concentrations by 66% and reduced the predialysis in vitro half-life of AcSDKP (270 +/- 109 minutes) to values (182 +/- 67 minutes) not significantly different from those of controls or nD patients. Most HD patients treated with ACE inhibitors had AcSDKP levels greater than 24 pmol/mL (the greatest concentration found in other nD and HD patients). Only in this group of patients did weekly doses of rHuEPO correlate with AcSDKP levels. Our results show that renal function is essential to maintain stable AcSDKP plasma levels, and at high levels, AcSDKP acts as a uremic toxin causing partial resistance to erythropoietin and inhibiting erythropoiesis.