Abstract: A pick-resistant-multiple-pin-tumbler cylinder for a lock is disclosed, the principal embodiments of which include an integral, key-displaceable, booby-trapped, lock-up element within the cylinder shell, and slidably mounted in relation to a portion of the rotationally displacable cylinder plug and in which the lock-up element contains a T-shaped peripheral notch which engages and receives a fixed pin or lug projecting from the bore and is thus both slidably and rotatively received in the shell and spring pressed axially for engagement of its shaped notch with the pin, whereby the peripheral dimension of the notch limits the rotation of the plug to a small angle until the lock-up trapping element is thrust towards the rear of the cylinder through the insertion into the plug's keyway of a properly profiled key, imparting a predetermined linear displacement effecting disengagement of the lock-up element.