Glucokinase from baker's yeast has been purified to homogeneity. The molecular weight of the subunit is 51,000. The native enzyme sediments with S20,w values in the range of 19 to nearly 4S. The presence of glucose and phosphate favors the heavier species while ATP causes depolymerization. Titration experiments with the Ellman reagent support this view. The enzyme subunit has four sulfhydryl residues of which one is more reactive than the other three. However, it does not seem to be directly responsible for the catalytic activity. The amino acid composition of the enzyme is similar to those of the hexokinases P1 and P2 but for aspartic acid and histidine.