In this study, lectin-conjugated gold nanoparticles (GNPs) were prepared by standard biotin-streptavidin chemistry. The lectin-conjugated GNPs can be used as an indicator for studying the interaction of lectin with glycosyl complex on living cellular surfaces due to the high affinity of the lectin with saccharides. The interactions of two well-known lectins (Ricinus communis agglutinin and concanavalin A) and three different cell lines (HeLa, 293, and 293T) were selected here to establish this assay. Highly binding affinity of R. communis agglutinin with cells was demonstrated by conventional microscopic and UV-visible spectroscopic studies. In addition, the binding process can be inhibited by galactose, giving further proof of the binding mechanism.