Thoracic duct lymphocyte depletion, a technique that selectively suppresses cellular immune defenses, was performed in a patient with myasthenia gravis. The patient demonstrated a 70% reduction in anticholinesterase medication requirement. These data lend further support to the apparent relationship between myasthenia gravis, the thymus, and cell-mediated immunity and implicate the lymphocyte, or a substance bound to it, as a contributing factor in myasthenia gravis.