The soluble adenosine diphosphate glucose-starch glucosyltransferase of maize (Zea mays L.) endosperm uses adenosine diphosphate glucose as a sole substrate, but the starch granule-bound nueleoside diphosphate glucose-starch glucosyltransferase utilizes both adenosine diphosphate glucose and uridine diphosphate glucose. The soluble glueosyltransferase can be bound to added amylose or to maize stareh granules that contain amylose. However, binding of the soluble enzyme to the starch granules does not ehange its substrate specificity to that of the natural starch granule-bound glucosyltransferase. Furthermore, the soluble gtucosyltransferase bound to starch granules can be removed by repeated washing without a change in specificity. The bound glueosyltransferase can be released by mechanical disruption of starch granules, and the released enzyme behaves in a manner similar to that of the bound enzyme in several respects. These observations suggest that the soluble and bound glucosyltransferases are different enzymes. The starch granule-bound glucosyltransferase activity is linearly proportional to the number of Wx alleles present in the endosperm. This is compatible with the hypothesis that the Wx allele is a structural gene coding for the bound glucosyltransferase, which is important for the normal synthesis of amylose.