A testing body (1) that is provided for verifying the penetration properties of a sterilization agent has, with a compact design, a particularly high detection sensitivity and is thus particularly well-suited or use in the sterilization of minimally invasive surgical instruments that are known to be difficult to remove air therefrom thereby rendering the sterilization thereof problematic. For this purpose, the testing body (1) has a detector volume (24), which is provided for accommodating an indicator (26) and which can be connected to a sterilization chamber via a gas collecting space (4). According to the invention, the gas collecting space (4) has a multistage design, and the cross-section and the volume of each stage (14, 16) decrease between adjacent stages (14, 16) in the direction of the detector volume (24).