This chapter discusses the preparation of glycopeptides and oligosaccharide probes for receptor studies. The majority of studies directed at examining the specificity of mammalian lectins have relied on monosaccharides and glycoproteins as probes. Glycoproteins yield results that are difficult to interpret because they generally bear multiple oligosaccharides of differing structure. Polyvalency, because of the presence of multiple oligosaccharides on a single polypeptide, adds another complicating feature to specificity studies. It is possible to circumvent these difficulties by utilizing characterized glycopeptides and oligosaccharides to examine the specificity and other properties of these lectins. The chapter describes the general methods for the preparation of both glycopeptides and oligosaccharides that can be radiolabeled and some of their applications. 14C and 3H can be introduced into glycopeptides by acylation with 14C or 3H-labeled acetic anhydride; however, the products obtained have a relatively low specific activity and require scintillation counting for detection. The introduction of a p-hydroxyphenyl moiety onto the amino terminus of glycopeptides makes it possible to prepare 125I-labeled derivatives of much higher specific activity.