A procedure is described for the determination of plasma or serum haemoglobin employing the peroxidase activity of the haemoprotein using 2,2'-azino-di-(3-ethyl-benzthiazolinsulphonate-6) as chromogen. The method gives equal results for free haemoglobin, methaemoglobin and haemoglobin complexed to haptoglobin. It is designed to measure haemoglobin in the range 0-12 μmol/l. The peroxidase activity of myoglobin is similar to that of haemoglobin, whereas haemin in free solution, bound to haemopexin or to albumin (methaemalbumin) shows much lower activity. The precision within run is satisfactory, ±5%.