Patients age 18 years and older who have had dermatomyositis for at least 6 months and who have not improved with prednisone or other therapies, or who cannot tolerate prednisone or other therapies, may be eligible for this 12-week study. Candidates will have a history and physical examination, including blood and urine tests, throat culture, and muscle strength testing. Participants will be randomly assigned to receive either h5G1.1-mAb or placebo (an inactive substance). The drug or placebo will be given intravenously (through a thin tube inserted into a vein) once a week for five doses and then every other week for two more doses.