Abstract: A locking mechanism for a plurality of interconnected beam members and upright members used in a rack or shelf system. For rack assembly, headed lugs on beam ends are slid into selected key-hole slots on the uprights. The locking mechanism uses a spring-loaded, captive pin translatable from a locked position to an unlocked position, and rotatable from an enabled position to a disabled position. When enabled and urged into a locked position, the pin enters an upper portion of the key-hole, vacated by the lug head when the beam and uprights are interconnected. The pin thereby prevents the lug head from being removed from the key-hole. Both automatic and manual operation of the locking mechanism is possible.