Chronic erysipelas polyarthritis in pigs is considered to be an animal model closely resembling human rheumatoid arthritis. In an experimental study using SPF pigs infected with Erysipelothrix rhusiopathiae several immunopathological aspects were studied for up to 14 months after infection. After an initial febrile phase a progressive polyarthritis and discospondylitis developed at 2 weeks after infection. Quantitative determination of IgG and IgM revealed a significant increase in these immunoglobulins both in the serum and the synovia from arthritic joints. Arthritic joints containing viable Erysipelothrix bacteria at termination of the experiment have a higher content of IgG and IgM than bacteriologically negative joints. During the entire experiment the rheumatoid factor titer, determined by the Waaler Rose test, remained relatively low with increases and decreases. The Erysipelothrix antibody titers are highly significant. The total complement activity in the serum increases discontinuously and shows fluctuations similar to those of the rheumatoid factor titer. The highest enzyme activities of acid phosphatase and lactate dehydrogenase are demonstrable in the synovial fluid of arthritic joints with positive bacteriological or immunohistological findings. These joints also have the greatest numbers of IgG-producing plasma cells. By means of immunohistological methods Erysipelothrix antigen can be demonstrated in macrophages of the proliferating synovial membrane. Finally, there is a discussion of the pathogenic significance for the perpetuation of chronic polyarthritis of Erysipelothrix persisting in arthritic joints in spite of focal immune reactions.