Synthetic and semisynthetic carbohydrates were used to probe sugar combining sites of animal lectins. For example, using derivatives of monosaccharides, it was determined that the 3-OH and 4-OH groups are absolutely essential and the 6-OH group was dispensable €or both mammalian and chicken hepatic lectins. When the glycosyl units are clustered into a di- or tri-branched structure, the binding affinity increased dramatically. This phenomenon is less pronounced when the lectin is ttsolubilizedtl with detergent. Rat and chicken hepatic lectins appear to have a similar geometric dimension of binding sites. The optimal binding appears to require terminal sugar residues to be separated by ca. 1.5-2.5 nm. More exact probing of rat hepatic lectin topography was performed with three derivatives of a tri-branched glycopeptides specifically derivatized with a photoactivatable group at the different terminally exposed Gal residues. The results of photoaffinity labelling showed that the rat hepatic lectin binds the tribranched oligosaccharides with a specific steric orientation.