A phenotypic analysis on synovial fluid cells from the carpal and tarsal joints of sheep with visna virus-induced inflammatory synovitis was performed. The results showed increased representation of cells bearing lymphocyte, macrophage, and dendritic cell markers compared to equivalent synovial fluid cells from normal uninfected age-matched controls. In infected sheep, CD8+ T cells tended to predominate over CD4+ cells, while the numbers of gamma delta T cells varied from being absent in some samples to constituting the major T cell subset in others. B cells were found in relatively smaller numbers. Analysis of the large mononuclear cells showed that they stained with monoclonal antibodies that recognize macrophages and afferent lymph dendritic cells. Major histocompatibility complex (MHC) class II+ macrophage/dendritic cells were found in normal joints, but significantly elevated proportions of such cells were present in the carpal joints of infected sheep. The intensity of MHC class II staining was also significantly elevated in infected animals compared to control animals. A high proportion of these cells also stained for CD1 in both normal and infected animals, but were significantly elevated in number in the carpal joints of infected sheep. The elevated proportion of cells expressing molecules associated with accessary cell function and the increase in the numbers of accessory molecules per cell suggests an enhanced capacity for presenting antigen to a variety of T cell subsets within the joints of infected sheep, which could initiate or perpetuate potentially damaging local synovial inflammatory responses.