Abstract: A lockout device and method of operation for enclosing, protecting and inhibiting a heat-activated fire extinguisher sprinkler head in a workplace during construction and maintenance therein. The device has two hinged semi-cylindrical clamping elements which clamp on to the end of a sprinkler supply pipe without tools, and has two hinged shell portions defining a cavity for protectively enclosing the said sprinkler head. Captive bolts and thumbscrews provide hand operated securing means to hold the clamping means and shell portions together, and a padlock provides locking means to prevent unauthorized removal of the device. Visible indicia means are provided for informing persons in said workplace of the presence and purpose of the sprinkler head lockout devices.