Our recent studies of the pathogenesis and manipulation of adjuvant arthritis are based on longterm lines and clones of T lymphocytes that mediate, prevent, or treat arthritis. The development of lines of T lymphocytes that are functional in disease is a technological advance that has cast new light on 3 aspects of adjuvant arthritis: its etiology, pathogenesis, and treatment. We believe this investigation of the animal model provides both tools and concepts that may be useful for rheumatology in general. Adjuvant arthritis begins as a subacute inflammation that progresses to a chronic polyarthritis; it was first described 3 decades ago by Pearson (1). The disease is inducible in genetically susceptible rats (2) by injecting them with Mycobacterium tuberculosis (Mt) in oil, a material known as Freund’s complete adjuvant (FCA). The arthritis, which appears about 11-13 days after injection, is prominent in the small joints of the extremities and is characterized by inflammation of the synovium, formation of pannus, destruction of cartilage, and erosion of bone (3). Acute