Enzyme-substrate interactions are comparable to lectin-carbohydrate or antigen-antibody interactions in terms of specificity. The dissociation constant (KD) values of lectins and enzymes are comparable (generally from millimolar to micromolar). Despite the enormous potential of specific interactions, enzyme-substrate interactions have generally not been exploited as a source for the construction of specific molecular probes, such as lectins. Polysialic acid (polySia) is a developmentally regulated carbohydrate polymer involved in neural cell differentiation, organogenesis, and malignancies. PolySia represents an important developmental antigen, and its specific detection method could be utilized to study neural plasticity, various malignancies, and central nervous system infections. Inactivated endosialidase represents a new approach to detect polySia, which due to the poor immunogenicity has been a complicated target for the production of antibodies. This chapter describes a catalytically inactive bacteriophage-derived endosialidase that can be used as a probe with lectin-like properties in the specific detection of its substrate, polySia.