The invention relates to a method for checking the efficacy of an exhaust gas after-treatment device (9), especially of a catalyst, for an internal combustion engine, exhaust gas generated by a donor internal combustion engine (2) flowing through the exhaust gas after-treatment device (9) to be checked. In order to allow the efficacy of an exhaust gas after-treatment device (9) to be checked with a high degree of flexibility and in a simple manner, a partial quantity is withdrawn from the exhaust gas flow of the donor internal combustion engine (2) and is fed to a defined partial inlet cross-section (22) of the exhaust gas after-treatment device (9) to be checked.