Platelet degranulation is a characteristic feature of platelet response to aggregating agents, but the mechanism and route by which secretory organelles are transferred to plasma are still uncertain. In the present study, human platelets were incubated with cytochalasin B, an agent which stabilizes discoid shape, and trypsin, which is known to cause release reaction and degranulation. Platelets treated in this manner retained their disc form, but were nearly devoid of granules and dense bodies. Electron-dense tracers indicated that degranulation was accomplished by fusion of secretory organelles with channels of the open canalicular system. The degranulated discoid platelet appears to survive exposure to cytochalasin B and trypsin and may prove to be a useful model for in vivo and in vitro experimental studies.