Morphological aspects of the platelet release reaction induced by adenosine 5′-diphosphate (ADP) and/or teleocidin (a tumor promoter) in the presence of aspirin were studied by transmission electron microscopy. Human platelet-rich plasma treated with reagents at 37 °C for 1–5 min was fixed with aldehyde and embedded in Epon or Lowicryl K4M. Addition of ADP (10 μM) resulted in the centralization of granules without granule release, while teleocidin (100 ng/ml) induced the swelling of the open canalicular system (OCS) and the release of α-granules without centralization of these granules. When both ADP and teleocidin were added successively, a synergistic effect was predominant; and both α- and dense-granules were discharged from the cytoplasm. Postembedding immunocytochemical studies employing specific antibodies against various granular components in combination with protein A gold as a label revealed that, following the administration of teleocidin, several components of α-granules such as β-thromboglobulin and platelet factor 4 moved into the swollen OCS, while the dense-granule component, serotonin, did not. When both teleocidin and ADP were administered, both α-granular and dense-granular components moved into the swollen OCS.