The gene for erythropoietin, the first growth and differentiation factor to be identified, has now been cloned and a recombinant erythropoietin is ready for clinical trials. This molecular achievement has also led to the identification of its mRNA in liver and especially in kidney tissue and here in the extra glomerular fraction. A radioimmune assay has been developed and its shows, as anticipated, low levels of erythropoietin in patients with kidney disease. However, in all other anemias, including the anemias of cancer, the levels of erythropoietin do not seem to be affected by the kind of disease but only by the degree of anemic hypoxia. The action of recombinant erythropoietin appears to be directed at surface receptors which increase in density on progenitor cells as they mature from early BFU-E to late CFU-E. These findings have led to an updating but not to a radical change in our concept of the feedback circuit which controls red cell production and the size of the red cell mass.