A method for determining the amount of glycated haemoglobin (HbA1c), in which—if required—the erythrocytes in a sample are haemolysed, the haemoglobin that is then released—if required—is contacted with a proteolytic agent and the glycated haemoglobin degradation products obtained in this way or otherwise are quantified is disclosed. In order to provide such a process and reagents employable therein that has/have the property of sufficient stability of the chemical compounds that are essential to the reaction, for the stabilization of the haemoglobin which is unfolded at a very low pH in the range from 1 to 3, at least one suitable stabilizer is present in the haemolysis solution. Where a leuco dye is used in connection with the determination of the amount of HbA1c, it is proposed that the latter be stabilized with particular phosphine compounds and/or thio compounds, and, in particular embodiments, the requisite proteolytic agent is to be provided in the form of an inactivated protease which is then only reactivated in situ.
