This chapter discusses the identification, assay, and chemical synthesis of isodityrosine. Isodityrosine is an oxidatively coupled dimer of tyrosine, with the two tyrosine units linked via a diphenyl ether bond. The biosynthesis of isodityrosine probably proceeds by the action of peroxidases on tyrosine, as indicated by the fact that peroxidase inhibitors block the incorporation of tyrosine into isodityrosine. Plant cells differ widely in their content of wall glycoprotein, and therefore isodityrosine. Cells with only primary walls are the richest sources and suspension-cultured cells are particularly well endowed. Because there are no specific assays for isodityrosine, it must be purified chromatographically before quantitation. Pure isodityrosine can then be assayed by its UV absorbance or by reaction with Folin and Ciocalteu's phenol reagent. The staining can be done either before or after elution from the chromatogram. Staining before elution is less accurate but simpler because the position on the chromatogram of the isodityrosine to be assayed does not have to be accurately deduced before visualization.