Stromelysin-1 (MMP-3) is an important member of the matrix metalloproteinase family. In joint-degrading diseases like arthritis, elevated levels of MMP-3 protein are detected in synovial fluid using immunological methods. However, these methods do not discriminate between active and inactive enzyme. In the present study, a specific stromelysin activity assay was developed using the selective fluorogenic substrate TNO003 (Dabcyl-Gaba-Arg-Pro-Lys-Pro-Val-Glu / Nva-Trp-Arg-Glu-(EDANS)-Ala-Lys-NH2, / =cleavage site). For its use in biological media, cleavage of TNO003 by enzymes other than stromelysin was effectively blocked by a proteinase inhibitor cocktail. Spiking of MMP-3 to synovial fluid resulted in an MMP-3 concentration-dependent linear increase in activity. The measured MMP-3 activity was not affected by the addition of MMP-13, even in a 5-fold excess over MMP-3. Synovial fluid from rheumatoid arthritis patients demonstrated 100-fold higher levels of active stromelysin than control synovial fluids.