Five Japanese Black cattle showing the tendency of persistent hemorrhage were diagnosed as δ-storage pool deficiency because of lack of dense bodies in platelets. There was no significant difference in the platelet count, fibrinogen concentration, prothrombin time and activated partial thromboplastin time between the cases and normal control cattle. However, the maximum platelet aggregation rate and the values of myosin in the cytoskeletons during platelet aggregation induced by collagen were significantly lower in the cases compared with those in normal control cattle. The quantities of platelet membrane glycoprotein were in the range of 94-160 kDa and were not different between the cases and control cattle. However, a decrease of thrombospondin in a-granules in platelet cytoplasm were suspected in two of the 5 cases