OBJECTIVE To determine the functional response of synovium to infection, and the influence of infected synovium on articular cartilage metabolism. METHODS Synovium and articular cartilage explants from the midcarpal and tarsocrural joints of adult horses. METHODS For experiment 1, synovium explants were incubated as follows: control--incubation in standard medium, infected (I)--incubation with Staphylococcus aureus, and infected-filtered (IF)--incubation with medium collected from the infected group and filtered (0.22-micron filter). Daily collected medium was assayed for interleukin 1 beta (IL-1 beta), IL-6, tumor necrosis factor, and hyaluronan (HA) concentrations. For experiment 2, cartilage explants were incubated as follows: control--incubation in standard medium, and IF--incubation in medium collected from infected synovium cultures and filtered. After 48 hours, explant proteoglycan synthesis and endogenous proteoglycan and glycosaminoglycan contents were determined. RESULTS IL-1 beta and IL-6 values were significantly increased in synovium explants from the I and IF groups. Hyaluronan concentration was lower in I and IF groups. Proteoglycan synthesis and content, and total glycosaminoglycan and chondroitin sulfate concentrations, were significantly decreased in cartilage from the IF group. CONCLUSIONS Bacterial infection was associated with decreased HA concentration and increased mediator release. These effects were also observed despite elimination of bacteria. Exposure to sterile but previously infected medium decreased articular cartilage matrix synthesis and composition. CONCLUSIONS Resident synovial cells may contribute appreciably to articular damage during bacterial infection in the absence of migrant inflammatory cells. This response is prolonged despite elimination of the bacteria.