SUMMARYThere is strong circumstantial evidence to support the concept that local microbial antigens play a key role in the synovitis of reactive arthritis (ReA) patients. It is not at all clear whether these antigens reflect the sequelae of previously viable organisms once resident in the joint. To address the microbicidal activity of synovial fluid (SF) we performed quantitative cultures of arthritogenic organisms (Salmonella typhimurium, Shigella flexneri, Klebsiella pneumoniae) and controls (Escherichia coli, Staphylococcus aureus) in the presence of SF from patients with ReA. There was a dramatic inhibitory effect of SF on the Gram-negative organisms (mean 1 . 35