Here we describe a new principle for accessing the activity of the different members of the human matrix-metalloproteinases (MMPs) by a colorimetric assay. Using protein engineering, a modified pro-urokinase was made in which the activation sequence, normally recognized by plasmin (ProArgPheLys/IleIleGlyGly), was replaced by a sequence that is specifically recognized by MMPs (ArgProLeuGly/IleIleGlyGly). The active urokinase resulting from the activation of this modified pro-urokinase by MMPs can be measured directly using a chromogenic peptide substrate for urokinase. The assay has been made specific for MMP-9 using an MMP-9 specific monoclonal antibody. Using this antibody MMP-9 is captured from biological fluids or tissue culture media, and MMP-activity of both active and latent MMP-9 can be analysed. We determined the gelatinase-B (MMP-9) activity present in saliva from patients with Sjögren's syndrome. Using a general gelatinase assay with radioactively-labeled gelatinated collagen it was observed that gelatinase activity was slightly, though not significantly, increased in patients: general gelatinase activity in patients versus healthy controls: 17.0 +/- 4.9 vs 12.2 +/- 2.5 x 10(4) cpm/ml (p > 0.05, and 44.0 (4.0 vs 36.1 +/- 1.9 x 10(4) cpm/ml (p > 0.05), for active and latent gelatinase, respectively. However, using the immunocapture activity assay (using modified urokinase) specifically MMP-9 activity was measured, which was significantly increased in saliva from patients compared to healthy controls: MMP-9 (already active): patients 8.9 +/- 2.5 U/mg, controls 1.0 +/- 0.5 U/mg (p = 0.002); latent plus active MMP-9: patients 53.1 +/- 9.8 U/mg, controls 16.5 +/- 2.6 U/mg (p = 0.01). This assay, measuring MMP-9 activity using modified pro-urokinase as a substrate can easily be adapted for the specific detection of the various members of the MMP-family or other difficult to measure proteases, in a format that can be used for high throughput screening of compounds or samples.