In bone-marrow tissue cultures of human and guinea pigs the author carried out observations on various degenerative changes of megakaryocytes with a phase-contrast microscope, and obtained the following results: 1. The degenerative changes in the cytoplasm of megakaryocytes are classified into 14 types; namely, granule abnormality, secretion granule formation, vacuole formation, bleb formation, spherical protrusion, small tongue-like processes, bubble formation, hyaline degeneration, honeycomb degeneration, membranous extension, thread formation, myelin formation, the swelling of the cytoplasm, and advanced degenerative group (coarse and uniform degenerations). 2. The degenerative changes in the nucleus of megakaryocytes are calssified into 10 types; namely, chromatin mass formation, brilliant nucleus, pyknosis, nuclear necrosis, nuclear lobe fusion, nuclear swelling, nuclear edema, caryolysis, dissociation of nuclear sustances, and naked nucleus. 3. The early symptoms of degenerative changes in megakaryocytes are abnormalities of the specific granules in cytoplasm, especially the increase in the brilliancy. 4. The vacuole formation is one of the degenerative changes most frequently encountered in megakaryocytes. 5. The brilliancy of the nculeus is one of the nuclear degenerative changes most frequently observable as well, and it often precedes the pyknosis and caryolysis, indicating the severity of nuclear disturbnaces.