Abstract: The rotatable plug of a cylinder lock has an axially extending outwardly open cavity paralleling a key slot and normally registering with an axially extending groove in the inner wall surface of the surrounding cylinder, the groove and the cavity being bridged by a locking bar with sloping flanks cammable radially inwardly, against the force of compression springs within the recess, upon rotation of the plug relative to the cylinder. Seated in the cavity are a plurality of axially spaced tumblers which are freely rotatable about pins transverse to the cylinder axis and have recesses alignable, in a predetermined operating position, with respective projections on the locking bar to enable the inward camming of the latter; each tumbler carries a pair of permanent magnets confronting a thin partition between the cavity and the key slot.