Summary:A metalloproteinase with a specificity for gelatin was isolated from serum-free medium of cultures of rheumatoid synovial fluid. The enzyme showed all the properties of a leukocyte gelatinase. In addition to gelatin this proteinase cleaved the synthetic Substrate dinitrophenyl-Pro-Gln-Gly-Ile-Ala-Gly-Gln-jD-Arg (Dnp-peptide) rapidly, while casein was a müch poorer Substrate. This proteinase showed no enzymatic activity against collagen type I, was secreted in a latent form and could be activated by trypsin or organomercurial compounds, such äs mersalylic acid or 4-aminophenylmercury acetate. The latent enzyme had an apparent molecular mass of 130000 -150000 estimated by gel filtration or 97000 by electrophoresis on polyacrylamide gel containing sodium dodecyl sulphate. When analysed by immunoblotting the enzyme was recognized by antibpdies raised against human polymorphonuclear leukocyte gelatinase. Although we found synovial fibroblasts to be largely present in the cell cultures we could not detect any fibroblast gelatinase activity.