The effectiveness of recombinant human erythropoietin (r-HEPO) was tested in 15 haemodialysis patients. The dosage was started at 24 IU/kg three times weekly, as an intravenous bolus at the end of the dialysis session, and then doubled every two weeks as long as the rise in haemoglobin was less than 2 g/dl. During treatment the reticulocyte count rose from 31 +/- 5 x 10(3)/microliters to 152 +/- 11 x 10(3)/microliters after 16 weeks. The haematocrit rose from 0.24 +/- 0.01 to 0.36 +/- 0.002. At the beginning of treatment the haemoglobin level was 7.3 +/- 0.3 g/dl and rose during treatment to 11.3 +/- 0.2 g/dl. Three patients developed hypertension and in two their Cimino shunt closed. but there were no toxic side effects, organ damage, allergic reactions or antibodies against the hormone. The results show that the anaemia of patients on chronic dialysis can be treated effectively and without serious side effects with r-HEPO.