A method and an apparatus for testing the piston (2) of an injection pump of a pressure medium, the piston comprising a longitudinal groove (2b) that begins from its frontal face (2a) and extends on its side, and a screw-shaped guide edge (2c) that abuts on the envelope surface of the piston. According to the invention, the piston (2) is inserted, with its frontal face (2a) first, into a testing cylinder (1), wherein the clearance between the piston (2) and the testing cylinder (2) is tight and wherein a specific testing pressure is exerted on the envelope surface of the piston, in a limited area, from the vicinity of the said guide edge (2c) to a selected measurement point. The piston (2) is moved in the testing cylinder (1), so that the said measurement point moves, following the guide edge (2c). Changes in the testing pressure are monitored to observe any failures in the piston (2) and the said guide edge (2c).