Quantitation of the erythropoiesis with radio-iron (59Fe) was applied to 9 patients with untreated myelomatosis. The method included blocking of the 59Fe reutilization by injection of non-radioactive iron. There was no uniform pattern in the Fe-kinetics values. The Plasma Iron Turnover (PIT) and the Red Blood Cell Iron Turnover (RBCIT) varied from subnormal to values markedly increased above upper normal limit. The calculated average Mean Red Cell Life time (MRCL) of erythrocytes was just below normal range. The mean Marrow Transit Time (MTT) was normal in the patients, despite subnormal venous haematocrit, indicating insufficient stimulation of the bone marrow. The renal function, measured as 51Cr-EDTA clearance, was found positively correlated to the RBCIT (r = 0.78, P less than 0.05). The results suggest that the previously demonstrated relationship between anaemia and renal failure in patients with myelomatosis is caused mainly by an inability of the bone marrow to produce sufficient red blood cells under the stress of anaemia related to the degree of renal impairment.