Abstract: A pin tumbler lock barrel assembly is capable of preventing the lock from being operated by the use of a skeleton key or any other tool inserted into the key slot. The lock is comprised of an improved hollow lock casing having a key plug located therein and a specially designed sleeve that surrounds the lock casing that is relatively rotatable with respect thereto. The sleeve is comprised of two halves that are interconnected by spring-loaded fasteners. A crescent-shaped slot is provided on the key plug and is aligned with two radial through bores formed in the lock casing and one of the sleeve halves. An alarm system includes a switch assembly on the periphery of the sleeve which is aligned with the radial bore. A specially designed sliding element with a cam-shaped end surface cooperates with the lock casing allowing only the correct key to operate the lock mechanism.