We demonstrate that the B chain of ricin toxin preserves its lectin activity when expressed as a fusion protein on the surface of fd phage. Moreover, B chain, which folds into two topologically similar globular domains, can be dissected into amino-terminal and carboxyl-terminal domains to form single binding domains (SBDs) of B chain, each of which displays specificity for complex galactosides. The specific binding exhibited by the fusion protein of these SBDs was eliminated when amino acid substitutions Gly-46 in SBD1 or Gly-255 in SBD2 for native asparagine were introduced to alter key residues implicated in hydrogen bonding with substrate. These data demonstrate that it is possible to use a prokaryotic expression system to stably express and screen ricin B chain and its SBDs for sugar-binding mutants. Expression of ricin B chain on the surface of fd phage provides a method that can be used to efficiently select mutants with altered binding activities from a randomly generated library.