Objective: Study of clinicopathological changes of synovial fluid in bovine bacterial arthritis' Design: Cross-sectional studY. Animals: One hundered and seventeen cases of bovine arthritis and 48 clinically healthy cows. Procedure: Synovial fluid samples were collected from 1 17 abattorial cases of bovine arthritis and 48 clinically healthy cows in the Tehran province' The samples were cultured for isolation of mycoplasma and other gramnegative and gram-positive bacteria. Physical appearance and viscosity were noted and mucin clot test and the measurement of WBCs, total protein, glucose, ALP' AST and ALT were done. Statisticat analysis: Data were analyzed by one-way ANOVA, Duncan's multiple range test and chi-square. Results: Bacterial organisms were isolated from 40 cases (34.3%) of bovine arthritis. Bacterial spp. organisms were included mycoplasma (46.3%), bacterial (32.%), yeast sp.
(l4.l%) and a mixture of bacterial and my coplusma agents (6.9%).In the most cases of bacterial arthritis, viscosity and mucine clot test were ranged from moderate to poor' In spite of higher WBCs, ALT, ALP, AST and total protein values in the synovial fluid of infected cases the values of glucose was lower than control(P<0.05). Conclusion: ln 667o of infected cases no microorganism was isolated. Therefore interference of viruses and immunologic reactions should be notified. Significant alterations in WBCs, total protein, glucose, AST, ALT and ALP values of synovial fluid showed that these changes are very important and can be diagnostic.