Abstract: A freeze protection plug has latching balls engageable bayonet-fashion with grooves in a fitting. The balls are normally held in engagement with the grooves by a cam, but move inward to release the plug from the fitting when the cam is moved by a thermal actuator carried by the plug. The balls are retained in tapered openings in the wall of the plug to reduce jamming. Helical ramps in the grooves permit the use of a strong, reliable plug ejecting spring, and make manual insertion of the plug easy. Insertion is aided by the ribs of a plastic cap, which facilitate manual grasping and rotation of the plug. A paint shield ring prevents failures resulting from paint bridges adhering both to the plug and to the fitting. A unitary molded lanyard with two oval loops prevents loss of the plug. The seal on the plug is located on the liquid system side of the latching balls and isolates moving parts from the liquid system, and is of small diameter.