Recently we purified to homogeneity hyaluronidase from stonefish (Synanceja horrida) venom, for the first time from a marine source [Poh, Yuen, Chung & Khoo (1992) Comp. Biochem. Physiol., in the press]. In the present study the reaction products of the hyaluronidase purified from stonefish venom were analysed. It produced tetra-, hexa-, octa- and deca-saccharides as major end products, but not disaccharides. The structure of the tetrasaccharide product was determined by enzymic analysis, in conjunction with h.p.l.c. and by 1H n.m.r., as GlcA beta 1-3GlcNAc beta 1-4GlcA beta 1-3GlcNAc. Chemical shifts of the structural-reporter-group protons of the constituent monosaccharides for the tetrasaccharide have been assigned. The enzyme did not act on chondroitin sulphate or dermatan sulphate. The results indicate that the stonefish hyaluronidase is an endo-beta-N-acetylglucosaminidase specific for hyaluronate.