A polyclonal antibody has been raised against purified human fibroblast collagenase and characterised. This antibody has been used in combination with a monoclonal anti-TIMP antibody in a double antibody sandwich ELISA to measure TIMP-collagenase complex. The assay range was 5-50 ng/ml complex, quantitated in terms of the TIMP component. The assay can measure complex even in the presence of at least a 40-fold excess of free TIMP. The level of TIMP-collagenase complex has been measured in serial samples of synovial fluid from two patients with septic arthritis; high levels of complex are found in some samples, and the level of complex shows an inverse relationship with the level of free TIMP.