Photometric evaluation of serum activity of angiotensin-converting enzyme (ACE) making use of synthetic chromogenic substrate N-(3-[2-furyl]acryloyl)-phe-gly-gly (FAPGG) is analyzed. Spectral characteristics of FAPGG are investigated. The Burger-Lambert-Bar dependence is not observed in any of the working concentrations. Extension of the spectral weight band is associated with the appearance of the absorbency/concentration linear function for FAPGG solution. Under such conditions the registered intensity of the analyzer sign (delta OD/min) is in strong correlation with the spectral weight band. As a result, high inter-analyzer variations were observed. This impedes the standardization of the method and prompts the use of attested calibrators. Linear function of analyzer's sign and ACE level is theoretically achieved at FAPGG concentration 1.0 +/- 0.2 mM (Km = 0.35 +/- 0.05 mM, minimum saturation concentration 1.4 +/- 0.2 mM). Synthetic ACE inhibitors are recommended to rule out the interference.