Chronic polyarthritis was produced in inbred C3H/He mice after intraperitoneal (ip) administration of allogeneic thymocytes of inbred line BALB/c mice. The polyarthritis persisted more than 18 months without additional ip treatment. Histologically, synovial cell hyperplasia, granulation tissue formation, and inflammatory cell infiltration were seen. More than 6 months after the initial administration, range of motion of the affected joints was significantly restricted and the erosive destructions of joint cartilage were found roentgenographically. Thus, this experimentally induced arthritis proved to have many pathologic features in common with human rheumatoid arthritis.