Chemically sulphated glycopeptides (derived from pig duodenal mucosa) inhibited Clostridium perfringens neuraminidase (EC 220.127.116.11) activity in a pH-dependent manner. Analysis of inhibition kinetics data indicated that, although the enzyme inhibition could not be categorized into any of the classical types of inhibition, it could be interpreted as a function of the size and shape of the substrates used. The enzyme activity was inhibited by 86% and 40% when tested with bovine submaxillary-gland mucin (mol. wt. 4 x 10(5)-40 x 10(5) and N-acetylneuraminyl-lactose (mol. wt. 633) as substrates respectively. Presence of sulphated glycopeptide did not affect the binding of N-acetylneuraminic acid (mol. wt. 309), a competitive inhibitor of Vibrio cholerae neuraminidase, to the enzyme active site. The enzyme inhibition was thus considered to be due to steric hindrance as a consequence of the non-specific interactions between the enzyme molecule and polyanionic sulphated glycopeptide affecting the differential accessibility of the substrate molecules to the enzyme active site. The enzyme-inhibitor interaction could be suppressed by rapid and many-fold dilution of the reaction mixture, by concurrent addition of the inactive enzyme or by partial removal of the sulphate esters from the sulphated glycopeptide molecule by the action of Helix pomatia arylsulphatase (EC 18.104.22.168).