Childhood rhabdomyosarcoma was compared with Moloney sarcoma virus (MSV)-induced rhabdomyosarcoma of mice. By light microscopy, electron microscopy, and immunofluorescence for myosin the tumors were similar. In man the ultra-structure is distinctive enough to make electron microscopy an important technic in the diagnosis of this tumor. Virus particles typical of MSV and murine leukemia viruses were seen in the 12-day-old mouse tumor. Viral particles were not found in the human tumor. Particles measuring 110 mµ to 120 mµ in diameter were observed in one specimen of human tumor. We believe that these are electron-dense fragments of endoplasmic reticulum, not virus.

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Supported by USPHS research Grants CA 08480 and CA 07054 and training grant CA 05176 from the National Cancer Institute, NIH, and by American Lebanese Syrian Associated Charities (ALSAC).

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