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We have characterized the cellular content and some antibody reactivities in synovial fluid (SF) from DA rats with CIA. Since CIA is widely used as a model for RA, in which many studies concerning immune responses are performed on SF samples, we considered it important to describe the local, disease-causing immune reactions in CIA. At the peak of disease (day 22 after immunization), the major cell population in CIA SF was granulocytes (72%), but macrophages (17·9%), plasma cells (2·6%) and lymphocytes(7·7%) were also present. The CIA synovial membrane (SM) obtained at the same time was mainly infiltrated by monocytes, with granulocytes, lymphocytes and plasma cells also present. Cell populations in blood did not differ between arthritic and normal DA rats. Equally, high anti-collagen type II (CII) and rheumatoid factor (RF) levels could be detected both in SF and in sera. Notably, RF levels were also increased in normal DA rats. Moderate levels of anti-heat shock protein 65 kD (hsp) antibodies were recorded systemically in both normal and diseased animals. In conclusion, the cellular composition in SF and in SM are similar in rat CIA and in RA. The morphological differences between SF and SM that are characteristic for RA could also be demonstrated in CIA. The antibody data indicate systemic production of anti-CII and anti-hsp antibodies as well as RF, but they give no support for local production of these antibodies in the joints, which is the case in RA.