The isozyme distribution, kinetic properties and intracellular localization of hexokinase (ADP: l>hexose-6-phosphotransferase, EC 2.7.1.1) were studied in erythrocytes, blood platelets, lymphocytes and granulocytes. Soluble and particulate fractions were separated by a rapid density centrifugation method after controlled digitonin-induced cell lysis. In lymphocytes and platelets the major part of total activity was particle-bound (78 and 88%, respectively). In granulocytes and erythrocytes most of the hexokinase activity was found in the cytosol. All cell types, except granulocytes, contain mainly the type I isozyme. Platelets contain only type I hexokinase, while in lyphocytes a minor amount of type III is present in the soluble fraction (less than 10% of total activity). The major constituent of granulocytes is type III hexokinase (70-80% of total activity), the remaining 20-30% is type I hexokinase. Erythrocytes contain a multibanded type I hexokinase. The substrate affinities of the type I hexokinase do not differ significantly between the different cell types or between soluble, bound and solubilized fractions. Only soluble hexokinase from iymphocytes shows a slightly decreased K m apparent for glucose (P < 0.05).