Erythropoietin mRNA is detected primarily in kidney peritubular cells in response to hypoxia, and this tissue is the major adult source of the hormone. Erythropoietin can be assayed by in vivo or in vitro biological methods, or by radioimmunoassay, but only the in vivo assay can distinguish the most biologically active forms. The mature hormone consists of 166 amino acids and approximately 50% of the mature molecule (Mr 39,000) consists of carbohydrate. The gene is highly conserved among species studied, and is located on human chromosome 7, region q11-q22. Recombinant erythropoietin has been administered to haemodialysis patients and shown to increase haemoglobin levels, reticulocyte numbers and haematocrit. In transfusion-dependent patients, the need for regular transfusions was abrogated. Problems with hypertension have been noted in previously hypertensive patients, but the results of clinical trials with erythropoietin suggest that it will provide a valuable alternative therapy, for correcting some disorders of erythropoiesis.