The catalytic necessity of tyrosine residues in uridine diphospho- glucose pyrophosphorylase [E.C. 2.7.7.9] was investigated. Chemical modification of the pyrophosphorylase by N-acetylimidazole indicated that tyrosine residues were essential for activity. Approximately 23 of 112 tyrosines per molecule of 475,000 Daltons could be 0-acetylated. Solvent perturbation difference spectroscopy supported this number of exposed tyrosine side chains and in conjunction with chemical modification indicated that at least 11 to 12 tyrosyl residues per protein molecule are fully exposed. it her subst rate, uridine t riphosphate or uridine diphosphoglucose, afforded significant protection against inactivation by N-acetylimidazole. The significance of these tyrosine residues is discussed in terms of a quaternary subunit model for uridine diphosphoglucose pyrophosphorylase.