The T cell response to bacterial antigens plays a major role in the pathogenesis of reactive arthritis (ReA) following enteric infections with Yersinia enterocolitica. To study the antigen specificity of the T cells at the site of inflammation, the response of cloned T cells from the synovial fluid of 2 patients with ReA to partially purified antigens of Yersinia enterocolitica was determined. The clones showed different patterns of response to various fractions, indicating a multiclonal response to Yersinia antigens, and these specificities differed in the 2 patients. Some T cells were specific for Y. enterocolitica; some cross-reacted with other enterobacteria. Proteins of 14 and 19 kDa could be identified as target antigens for the T cell clones, but no clone could be unequivocally found that reacted with the highly purified Yersinia 61-kDa heat shock protein. Thus, the inflammatory T cell response in the synovial fluid in ReA is multiclonal and not predominantly directed against the bacterial heat shock 61-kDa protein.