The activity and specificity of a galactose-specific and a mannose-specific jacalin-related lectin from the bark of the black mulberry (Morus nigra) tree has been re-investigated using different experimental approaches. Both lectins definitely behave as polyspecific lectins recognizing galactose, mannose, and glucose even though MornigaG and MornigaM interact preferentially with galactose and mannose, respectively. The exceptionally extended size of the carbohydrate-binding site of both lectins apparently accounts for their polyspecific character. Parallel studies with other mannose-specific jacalin-related lectins confirmed that their exclusive specificity towards mannose/glucose relies on a reduced size of their carbohydrate-binding site. (C) 2003 Elsevier Science (USA). All rights reserved.