Abstract: The invention relates to an apparatus for destroying tumor cells or pathogens in the blood stream, comprising at least one ultrasonic frequency generator (11) and a device for forming an extracorporeal blood circulation (1) which encompasses at least one heat exchanger (3), at least one blood circulation pump (2, 2?), and a treatment vessel (6) for holding blood. The treatment vessel (6) is mounted downstream of the heat exchanger (3), forms a treatment chamber (13), and is connected to at least one ultrasonic transducer head (8) which is coupled to the ultrasonic frequency generator (11) such that a low-frequency ultrasonic vibration can be introduced into the treatment chamber (13).