A unique case history is presented, of a 42-year-old patient who has suffered three episodes of a demyelinating neuropathy, each of which followed an injection of tetanus toxoid. The clinical features on each occasion were characteristic of acute idiopathic polyneuropathy; a rapid onset of a mainly motor neuropathy with eventual recovery. Nerve conduction studies performed during the second and third episodes demonstrated grossly slowed motor conduction velocities. The sural nerve was biopsied after the third episode, and the features seen on light and electron microscopy included prominent hypertrophic changes, mononuclear cells associated with most "onion bulbs" and macrophage mediated demyelination. Studies of blastogenesis and macrophage migration inhibition, showed T lymphocyte responsiveness to both peripheral nerve myelin and tetanus toxoid. Typing for antigens of the HLA system indicated that the patient was homozygous for HLAB8.