The levels of metalloproteinases and metalloproteinase inhibitors were measured in rheumatoid synovial fluid. Reliable estimates of total enzyme and inhibitor levels in the synovial fluids were obtained only after hyaluronidase treatment and gel filtration. Three latent metalloproteinases were found which, after activation, degraded collagen, proteoglycan, and gelatin. These enzymes closely resembled the metalloproteinases secreted into connective tissue culture medium. In addition to alpha 2 macroglobulin, an Mr 30,000 collagenase inhibitor was detected which closely resembled the tissue inhibitor of metalloproteinase found in tissue culture medium.