A novel lectin was isolated and purified from the culture filtrate of the soilborne plant pathogenic fungus Sclerotium rolfsii by anion-exchange chromatography using a DEAE-Sepharose column. The lectin came through the column with the flow-through, whereas all the non-agglutinating proteins present in the crude preparation remained bound to the column until elution in a NaCl gradient. SDS-PAGE analysis of the agglutinating fraction revealed a single band corresponding to a protein with a molecular mass of approximately 45 kDa. Agglutination of Escherichia coli cells by the purified lectin was not inhibited by any of the mono- or disaccharides tested, whereas the glycoproteins mucin and asialomucin did inhibit agglutination. Proteases, as well as 1,3-beta-glucanase, were found to be totally destructive to agglutination activity, indicating that both protein and 1,3-beta-glucan are necessary for agglutination. Using a biomimetic system based on binding of the lectin to the surface of inert nylon fibres revealed that the presence of the purified agglutinin on the surface of the fibres specifically induced mycoparasitic behaviour in Trichoderma harzianum. Trichoderma formed tightly adhering coils, which were significantly more frequent with the purified agglutinin-treated fibres than with untreated ones or with those treated with non-agglutinating extracellular proteins from S. rolfsil. Other mycoparasite-related structures, such as appressorium-like bodies and hyphal loops, were only observed in the interaction between T. harzianum and the purified agglutinin-treated fibres.