The rate of transfer of [32P]phosphate from [32P]-labeled phosphoglucomutase (alpha-D-glucose-1,6-bisphosphate:alpha-D-glucose-1-phosphate phosphotransferase, EC 2.7.5.1) to glucose increases dramatically between pH 8.5 and 10.5 with a half maximal rate at pH 9.8. This suggests the participation of a residue containing an ionizable group with a pK close to 10. The inhibition of enzyme activity obtained with tyrosine-derivatizing reactions--iodination, nitration, acetylation, and diazo coupling--is strongly indicative of tyrosine participation. Thiol reagents, p-hydroxymercuribenzoate and ethyleneimine, were without effect. Vanadate and arsenate augmented the transfer reaction 200- and 2.5-fold, respectively, and lowered the pH optimum of the reaction.