Polyneuropathy associated with anti-MAG monoclonal IgM gammopathy is responsive of mainly a sensory deficit in predominantly males ,aged 40-70 years. Significantly high serum anti-MAG antibodies are linked with demyelinating features of the peripheral nerves.Rituximab, an anti-CD 20 monoclonal antibody is a new drug which reduces B-lymphocytes. This study will test the safety and efficacy of rituximab in the treatment of patients with anti-MAG polyneuropathy.

Acting of a polyneuropathy for which there is not any treatment of reference today (see supra), a test of double-knowledge versus placebo is justified. Acting of a chronic polyneuropathy, the clinical evaluation must be led over one one year period. Acting of a sensitive polyneuropathy and the awaited benefit being the IMPROVEMENT OF the CLINICAL SIGNS, the principal criterion is a sensitive score in addition validated in chronic sensitive polyneuropathies immunodeficiency syndrome.

The patients answering the criteria of inclusion and none inclusion (see V-2) will be randomized in 2 groups: the first group will receive a perfusion IV of rituximab to the amount of 375 mg/m2 of body surface, once per week, during 4 weeks (see VII-3); the second group will receive 4 perfusions IV of placebo according to same methods'. The evaluation (see VI-1) will be carried out at the time of the randomization, then repeated in 3 months, 6 months, 9 months and 1 year.

Inclusion Criteria:

• IgM monoclonal gammopathy
• Anti-MAG antibody titers > 1.1000 BTU (ELISA)
• Worsening polyneuropathy with INCAT score > 4
• Informed consent

Exclusion Criteria:

• Severe comorbidity
• Other concurrent causes of polyneuropathy
• Concurrent immunosuppressive therapies (wash-out > 3 months)
• Previous treatment with rituximab
• Lymphoproliferative disease indicating other immunosuppressive treatment
• Unability to follow-up
• Previous documented side-effects with components involved in the tested drug
• White cell count < 1500/mm3 or platelet count < 75.000/mm3
• Patient under law