We evaluated eight patients from four families because of a history of excessive bleeding. Most patients had prolonged bleeding times, absent secondary wave of platelet aggregation in response to epinephrine, collagen and adenosine diphosphate (ADP), and defective 14C-serotonin release and platelet factor 3 availability. These findings are characteristic of a platelet release defect. Electron-microscopy examination of the platelets of seven patients revealed a common abnormality. From 30% to 70% of the platelets in any given sample exhibited a prominent membrane complex and dilated, tortuous surface-connected canalicular system ('swiss-cheese' platelet). In two patients there was coincident storage-pool disease, but the remainder had adequate dense bodies and a normal ratio of ATP to ADP. SDS-polyacrylamide gel electrophoresis of platelet proteins and glycoproteins showed no abnormalities. The patency of the canalicular system was demonstrated in one patient by the observation that dense bodies appeared in the cannaliculi and outside the platelets following collagen-induced aggregation of polylysine-treated platelets. Since platelet-aggregation responses to the calcium ionophore A23187 were normal, we conclude that the defective platelet function in these patients may be due to impaired calcium mobilization from the morphologically abnormal membrane complex.