Samples of synovial fluids aspirated from patients with septic arthritis prior to the commencement of any treatment contained active metalloproteinases but no proteinase inhibitory activity. We therefore assayed these samples for proteinase-inhibitor complexes. Although no biologically active alpha 2-macroglobulin or tissue inhibitor of metalloproteinase (TIMP) was present in the fluids, immunoassay of the samples clearly showed that high molecular weight proteinase-TIMP complexes were present. It is proposed that high levels of active metalloproteinases are released from neutrophils into septic synovial fluids and that these proteinases complex all the available TIMP, forming metalloproteinase-TIMP complexes.