Abstract: A fitting is associated with a closed fluid system for allowing testing by means of a probe, meanwhile insuring the system is maintained in a closed condition. Such fitting has a resilient rubber member one side of which is exposed to the fluid in the system, and the other side of which is exposed to outside the system. The rubber member defines a passage from one side to the other which is held closed by the rubber member mounting means, which compress the rubber member to so close the passage. A testing probe may be pushed into and through this passage, to be exposed to the fluid in the system. The resilient member is in sealing contact with the probe when it is so introduced, and upon removal of the probe, the passage again closes, insuring that the system is continuously in a closed condition.