This study investigates which factors influence the response of administered recombinant human erythropoietin (Re-HuEPO) with respect to the increase of haemoglobin in patients with end-stage renal disease. Pharmacokinetic parameters of administered Re-HuEPO in patients with end-stage renal disease and considerable differences in the amount of Re-HuEPO required ("Re-HuEPO-need") to obtain an increase of haemoglobin, revealed a pattern of dose-dependent first-order elimination without significant interindividual differences between the patients. As variable immunological inhibitors of erythropoietin are also absent, the administered Re-HuEPO seems to be equally available to the erythron in the various patients. In vitro incubation experiments with bone marrow cells show that the sera from patients with end-stage renal disease contain inhibitors of the erythropoietin-induced stimulation of bone marrow cells. As the patients' sera differ with regard to the degree of inhibition of erythropoietin bioactivity, this inhibition may also be responsible for the interindividual differences in amount of erythropoietin required. Besides a reduced endogenous production of erythropoietin, these inhibitors of the bioactivity of erythropoietin may also contribute to the pathogenesis of anaemia in patients with chronic renal failure.