Changes in the serum haptoglobin level have been reported in certain forms of anaemia (Allison and ap Rees, 1957; Allison, 1958; Nosslin and Nyman, 1958; Nyman, Gydell, and Nosslin, 1959), in hepatobiliary disease (Owen, Mackay, and Got, 1959) and in various inflammatory and chronic diseases (Jayle and Boussier, 1955; Allison and Blumberg, 1958). A simple colorimetric procedure for the determination of serum haptoglobins is presented. It is based on the peroxidase activity of haptoglobin-methaemoglobin complexes.