The ultrastructure and functions of platelets and megakaryocytes in 8 patients of a family with gray platelet syndrome were investigated. Hemostatic examinations on these patients revealed prolonged bleeding time, decreased platelet retention rates and decreased platelet aggregation rates induced by ADP, collagen, Ristocetin and epinephrine. Marked decreases in ATP and ADP release in response to these agents were also noted. Clinical and coagulation studies on this family suggested that the hereditary nature of the syndrome is autosomal dominant. Platelets and megakaryocytes in the peripheral and bone marrow blood smear from the patients showed peculiar gray color by May-Giemsa stain due to a deficiency of alpha-granules. Electron microscopic examinations revealed slightly enlarged platelets containing a deficient amount of alpha-granules, whereas dense bodies and mitochondria appeared normal. Several morphological abnormalities of patient's platelets, such as aggregates of dense tubular systems, circular arrays of dense tubular systems, an area of cytoplasmic sequestration with an enclosing membrane, clumps of dense material and remnants of Golgi apparatuses were recognized. Megakaryocytes showed normally developed Golgi zones, defective alpha-granule synthesis and liberation of abnormal platelets as shown in the peripheral blood smear.