Human synovial cells were cultured in vitro and tested for the activities of two lysosomal enzymes, cathepsin B and N-acetyl-beta-D-glucosaminidase (NAGA) under various conditions. Unstimulated synovial cells display intracellular and extracellular activities of both enzymes. However, cathepsin B was secreted in a latent pepsin-activatable form, whereas NAGA was secreted in an active form. Most of the cell strains analysed secreted rather limited amounts of the enzymes (less than 25% of total activity); some strains, however were highly secretory, the secreted activity reaching up to 50% of total activity. Cells were then stimulated with human recombinant interleukin-1 alpha (rhIL-1 alpha) or beta. Only the levels of secreted NAGA were clearly increased. Results are to be interpreted in view of the role played by synovial cells and by the lysosomal enzymes they release in inflammatory joint diseases and it would be worthwhile in the future to check for secreted NAGA in various body fluids, such as the synovial fluid of the inflamed joint.