The relative importance of erythropoietin (Ep) and inhibitors of erythropoiesis in the development of anemia in pediatric patients with end-stage renal disease (ESRD) was assessed in 82 patients: 41 treated with peritoneal dialysis (PD) and 41 with hemodialysis (HD). Serum Ep was determined with a sensitive radioimmunoassay. Potential serum inhibition of erythroid (CFU-E) and granulocytic (CFU-GM) progenitor cell growth was assessed using human bone marrow cell cultures. The mean Ep level for all 82 patients was 33.1 +/- 3.1 mU/ml, which was significantly higher (P less than 0.05) than the values obtained in 29 normal children (26.2 +/- 2.4 mU/ml). Serum Ep in the PD group (41.6 +/- 5.6 mU/ml) was significantly higher (P = 0.007) than that of the HD group (24.6 +/- 2.1 mU/ml). The mean hematocrit in the PD group (25.2 +/- 0.8%) was also significantly higher (P less than 0.002) than that of the HD group (22.2 +/- 0.5%). The mean serum parathyroid hormone (PTH) level as measured by a mid-terminal radioimmunoassay was not significantly different (P = 0.79) in the HD group (17,298 +/- 3,998 pg/ml) from that of the PD group (15,747 +/- 4,227 pg/ml). Neither serum Ep nor PTH concentration correlated with hematocrit or degree of inhibition of erythroid progenitor cell colony (CFU-E) formation in either group of dialysis patients, nor did the hematocrit correlate with the degree of serum inhibition of CFU-E formation. The higher level of Ep in the PD group may indicate more effective removal by PD of some enzymatic substance which reduces the immunologic and biologic activities of Ep.(ABSTRACT TRUNCATED AT 250 WORDS)