The invention relates to a process for the determination of the .gamma.-secretase activity, individual components of the process and the use of the process.The present invention relates to a novel process for the determination of the .gamma.-secretase activity and for the detection of .gamma.-secretase; particular embodiments of the process relate on the one hand to processes for the identification of a .gamma.-secretase or of a cDNA which codes for a .gamma.-secretase and on the other hand to processes for the identification of substances which can inhibit the activity of a .gamma.-secretase. Such substances have particular importance, as they can be used, for example, as pharmaceutical active compounds, e.g. for the treatment of Alzheimer's disease.