An implantable cardiac stimulation lead for implantation along the septal wall and/or the free wall of the left ventricular is disclosed. The lead is a microlead formed in its distal portion by a microcable with an active portion comprising a series of exposed areas forming the stimulation electrodes. This lead is implanted by an accessory with a needle having a puncture pointed free end and an opposite end mounted on a gripping end tip, and a releasable device for holding the microcable along the length of the needle. The microlead is introduced by injection of the microcable with penetration of the needle into the wall thickness of the interventricular septum or in the thickness of the free wall of the left ventricle, below the surface and along this wall between the apex region the atrial region.