A lockable quick-release mechanism for preventing theft of a bicycle wheel or seat from a bicycle frame. The lockable quick-release mechanism includes a rotatable lever having an eccentric cam rotating within a piston head. As the eccentric cam rotates within the casing, the piston head moves in relation to the casing. For tightening, the piston head is brought from its outermost to innermost position relative to its casing. Conversely, for loosening, the piston head is brought from its innermost to outermost position relative to its casing. Flat abutment surfaces are formed on the lever adjacent to the eccentric cam. A locking disc is disposed within the casing and includes a pair of extension arms engaging the abutment surfaces to prevent rotation of the lever. A spindle, which is disposed within the casing and extends through an aperture in the locking disc, rotates the locking disc from a locked to an unlocked condition. The spindle and locking disc are retained in the locked condition by a lock means.