The traditional treatment of inflammatory myopathies (IM) and generalized myasthenia gravis (MG) is immunosuppressive therapy, usually beginning with corticosteroids. However, up to 70% of treated patients show an incomplete response, including 10 - 30% who are unresponsive. Corticosteroids and other immunosuppressive therapies presented also many side effects. We propose to evaluate in a pilot, open, prospective, multicentric, phase II study, the interest of rituximab in the treatment of patients with primary IM associated with specific AAb (anti-synthetase and anti-SRP AAbs), or MG (with anti-AchR AAbs), refractory to conventional therapies. Twenty fourth patients with primary IM (12 with anti-synthetase, 12 with anti-SRP AAbs), and 12 with MG will be included in the study.