The mouse pigment mutant pale ear, ep/ep, which has a defect in kidney lysosomal enzyme secretion, had prolonged bleeding on experimental injury. Platelet counts and platelet protein did not differ from normal. There was, however, a deficiency in the platelet dense granule contents, serotonin, ATP, and ADP. Furthermore, a marked reduction of platelet dense granules was observed by electron microscopy. The results suggest that pale ear is a useful animal model in the study of platelet storage pool disease. Studies on this mutant and other pigment mutants have established that one gene can regulate at least three subcellular organelles, including the melanosome, the lysosome, and the platelet dense granule.