The involvement of hyaluronic acid (HA) oligosaccharides and blood-derived mononuclear cells in inflammatory processes prompted us to determine whether peripheral blood mononuclear cells (PBMC) possess hyaluronidase activity. PBMC were incubated with macromolecular-tritiated HA at pH 3.8 and supernatants were analysed by size exclusion chromatography to reveal digestion of HA. This digestion was due to the CD14-positive (CD14+), adherent, non-specific esterase-positive, subpopulation of PBMC. Hyaluronidase activity (72 kDa) was found in aqueous and non-ionic detergent PBMC extracts but not in the medium in which the cells had been cultured. These results indicate that hyaluronidase is, at least in part, linked to the membrane rather than excreted. Hence, monocytes have one or more hyaluronidases that can generate a pool of active HA fragments within tissues. Hyaluronidase activity was also found in 3/3 myelomonocytic lineage leukaemias but not in 3/3 lymphoblastic leukaemias.