The morphology of the blood platelets has been studied under various conditions by fixing the platelets, observing them under the phase microscope, and classifying them according to shape. It has been found that most platelets maintain their native disc form in citrated or oxalated blood kept at 37 C. In EDTA blood they are converted to spheres. The platelets in citrated and oxalated blood are sphered by 10 minutes' exposure to 0 C., but many of them resume the disc form when body temperature is restored. Washed platelets prepared from EDTA blood are spherical even when resuspended in saline or in gelatin solution and incubated at 37 C. for 45 minutes. When resuspended in oxalated plasma or in albumin or gum acacia solutions, however, many of them revert to the disc shape. A smaller proportion of unwashed platelets, separated from EDTA blood by centrifugation and resuspended in oxalated plasma also resume their disc form after incubation.