A medical instrument comprises a shaft which is connected at the proximal end to a handle, and it also comprises an insert which is guided along the shaft and which is connected at the proximal end to a movable grip part of the handle. At its distal end, the insert has a tool which can be actuated by pivoting of the grip part. A lock is also provided which can be brought into locking engagement with the movable grip part. To cancel the lock function, a detent is provided which can be brought between lock and movable grip part, such that the lock function can be cancelled by the detent.