Blood platelets from 10 normal human subjects have been examined with a sensitive differential interference contrast (DIC) microscope . The entire transformation process during adhesion to glass is clearly visible and has been recorded cinematographically, including the disk to sphere change of shape, the formation of sessile protuberances, the extension and retraction of pseudopodia, and the spreading, ruffling, and occasional regression of the hyalomere . The exocytosis of intact dense bodies can be observed either by DIC microscopy, or by epifluorescence microscopy in platelets stained with mepacrine . Details offluorescent flashes indicate that the dense bodies usually release their contents extracellularly, but may do so intracytoplasmically under the influence of strong, short wavelength light on some preparations of mepacrine-stained platelets . The release of one or more dense bodies leaves a crater of variable size on the upper surface of the granulomere . Such craters represent the surface component of the open canalicular system and their formation and disappearance can be directly observed . Because these techniques permit quantitation of several parameters of motility which are not readily observable by other techniques, it is suggested that high extinction DIC microscope examination may become a rapid and useful method of studying congenital and acquired platelet disorders . Many features of platelet transformation have been confirmed and extended by scanning electron micrographs . These can in turn be interpreted by reference to time-lapse films of living platelets.KEY WORDS platelets " transformation release reaction -exocytosis pseudopodia motility -DIC microscopy scanning electron microscopy It has been known since the last century that platelets (thrombocytes) undergo a change of shape (transformation) as part of their role in hemostasis (for reviews, see references 19, 20, 22, 51, 60, 66, and 67) . Light microscope observation established that platelets formed spiky protuberances and gradually spread on glass surfaces in imitation of their spreading on damaged vascular endothelia (9) . However, the resolution and sen-J . CELL BIOLOGY C The Rockefeller University Press "