Follicular fluid proteoglycans play an important role in human oocyte maturation, including the development of a fluid-filled compartment and maintenance of the hypocoagulative state of the follicular fluid. Human xylosyltransferase (EC 2.4.2.26, XT) is the key enzyme in the biosynthesis of glycosaminoglycan chains in proteoglycans and is secreted into body fluids together with large proteoglycans. We investigated the XT activities in human follicular fluid and granulosa-lutein cells from women undergoing IVF procedures. The mean XT activity was determined as 17.7 mU/l, which is 20-fold higher than in serum and the highest XT activity ever found in body fluids. Cultured human granulosa-lutein cells secreted large amounts of XT (14.52 micro U/10(6) cells), indicating that these cells are the main source of this enzyme in human follicular fluid. The XT from human follicular fluid was found to be associated with large chondroitin sulphate-containing proteoglycans. Furthermore, heparin was shown to bind strongly to the follicular fluid XT and to inhibit its enzyme activity. These findings indicate that XT may play a role in maintaining the haemostatic potential of the follicular fluid.