Transplantable murine neuroblastoma C 1300 was studied ultrastructurally at varying time intervals ranging from 2 hours to 40 days before and after X-irradiation. Following X-irradiation, 2000 and 4000 rads in a single dose, the uniformly small tumor cells became progressively enlarged multinucleated and degenerated, starting at one to two days. At five to seven days, the uni- and multinucleated giant cells predominated over the small tumor cells, while the giant cells progressively disappeared therafter and the small tumor cells predominated over the giant cells at 10 to 14 days. The giant cells contained abundant subcellular organelles and the X-irradiated tumor cells apparently continued to produce the organelles until they degenerated. Two types of cytoplasmic particles, intracisternal A and bar-shaped, were observed in the tumor cells. The intracisternal A particles occurred in almost all non-irradiated tumor cells though their number varied considerably from cell to cell, while they were observed less frequently in the radiation-induced giant cells probably due to a dilution effect rather than an actual numerical decrease. The bar-shaped particles, hitherto undescribed in the neuroblastoma, were 23 nm in diameter, variable in length and occasionally tubular. They occurred only in degenerating cells regardless of X-irradiation but were encountered more frequently in irradiated tumors than in non-irradiated ones. It is suggested that they may represent an unknown degenerative product of cytoplasm and/or nucleus rather than virus particles, despite their morphological resemblance to certain virus particles.