SEM images of rat peritoneal mast cells showed that microtubule-like filaments seemed to penetrate the granules and project several branches just beneath the granular surface. Each granule appeared to be a cluster of microgranules. TEM observations frequently revealed microtubules connecting the granule surface to the subplasmalemmal network or to the adjacent granule surface and on some occasions, microtubules also appeared to penetrate the granule membranes. Moreover, TEM revealed that individual granules seemed to consist of a mass of microgranules, each 10-15 nm in diameter; freeze-fracture images showed similar configurations. These findings suggest that the microtubule-like filamentous structures in mast cells play a role corresponding to that of the open canalicular system in platelets.