The endogenous production of 14CO during the first week after administration of glycine-2-14C (early labelled peak, ELP) was determined in 13 healthy subjects. The subjects were studied during normal, suppressed or stimulated erythropoiesis induced by hypertransfusion and phlebotomy, respectively. The production of 14CO was calculated from determinations of the specific activity of the body CO store and the endogenous production of CO. The incorporation of labelled glycine into circulating red cell haemo;globin haem was calculated from the maximal specific activity of haemoglobin haem and the total amount of Hb. The average ELP was 73.0 X 10(3) disintegrations per min (dpm) and the incorportion of glycine into circulating haemoglobin haem 491 X 10(3) dpm in controls. The ELP and the incorporation of glycine increased by 86% and 92%, respectively, in phlebotomized subjects and decreased by 37% and 62%, respectively, in hypertransfused subjects. A linear regression equation was calculated for the relationship between ELP and erythropoiesis, expressed as the incorportion of glycine into circulating haemoglobin haem. The residual ELP remaining in the absence of erythropoiesis was 24.9 X 10(3) dpm calculated from data of all the subjects, and 32.8 X 10(3) dpm calculated from data of controls and hypertransfused subjects only. In conclusion, erythropoietic haem turnover was found to contribute about 60% and nonerythropoietic (mainly hepatic) haem turnover about 40% of the ELP in man.