Morphologic and functional studies have implicated islet microtubules in the transport of the B-cell secretory product form the endoplasmic reticulum to the peripheral pool of secretory vesicles. The participation of the microtubular apparatus in the insulin release mechanism appears to be mediated through an increased rate of tubulin synthesis and of tubulin polymerization, two possible sites for a physiologic and pharmacologic regulation of hormone discharge. It is conceivable that cytoplasmic microtubules from either a rigid cytoskeleton which facilitates hormone transport by establishing n intracellular organization or act as a motion generating system along which the secretory vesicles are actively transported to the cell periphery. The existence of an eventual interaction between secretory vesicles and islet microtubules has been examined by measuring I123-tubulin binding to various subcellular fractions. In working out the experimental procedure on liver tissue, tubulin was found to bind to all subcellular fractions being most pronounced in the microsomial fraction; in the cytosol, tubulin was incorporated into high molecular weight complexes. Similar results were obtained with islet subcellular fractions, binding per microgram protein being tenfold higher than in liver tissue. In view of the calcium-induced increase in tubulin binding to islet subcellular fractions, and of the high affinity of tubulin and secretory vesicles for calcium, it is suggested that a calcium stimulated bridge - eventually microfilamentous in nature - might link the growing microtubules to the secretory vesicles and could, as such, participate in the intracellular transport of the secretory product.