The hydrolysis of maltose by glucoamylase (EC 3. 2. 1. 3 from Rhizopus niveus) was carried out in the presence and absence of dextran and dextran sulfate, which are the components of supports of immobilized enzymes. The interaction between dextran and the enzyme was observed by the fluorescence spectrophotometry. The kinetic and fluorescence experiments indicated that dextran bound to glucoamylase and was apparently a noncompetitive inhibitor of the enzyme. The dissociation constant of the enzyme-dextran complex was estimated to be 34%. The reaction rate was hardly affected at pH 4.0 and 4.5 by addition of dextran sulfate, while the kinetic parameters depended considerably on the concentration of dextran sulfate at pH 3.5. These findings indicated that there might exist some interactions between the enzyme and dextran sulfate.