The most satisfactory diagnostic procedure for hereditary angioneurotic oedema is the demonstration of low serum levels of C1 esterase-inhibitor. A modified method for the assay of this protein is described. It is based on the kinetic measurement of the C1 esterase-inhibitor when it inhibits the hydrolysis of N-acetyl-L-tyrosine-ethyl ester by C1 esterase. The relative C1 esterase-inhibitor concentration is based on the initial hydrolytic velocity, which can be evaluated from the pH change in a short time and within a small range. High reproducibility, cheap instrumentation and short time of analysis are some of the favorable aspects of this method in comparison with the 'end point titrimetric' method. Furthermore, this paper describes the mechanism of inhibition of C1 esterase by C1 esterase-inhibitor. The results are indicative of a non-competitive mechanism. The value of the Michaelis-Menten constant, Km, is 0.017 +/- 0.001 mol/l at 37 degrees C, in the optimum pH range 7.2-7.4. An estimate of KI in arbitrary units is also given.