Human tumour astrocytes (118MG) were exposed in vitro to the Engelbreth-Holm strain of Rous sarcoma virus at a multiplicity of infection of one. Morphological transformation of the cells in 60-mm plates was complete in 9-11 weeks after viral exposure. The transformed cells (EH-118 MG) grew as a monolayer, upon which nests of rounded cells developed. From these nests, rounded cells shed into the medium and in turn on seeding formed a monolayer and repeated the cycle. The transformed cells contained the viral group-specific antigen in the cytoplasm, and produced Rous sarcomas in chickens. No complete virus was demonstrated in the mammalian cells. Electron micrographs of the parent 118 MG and transformed EH-118 MG cells gave further details of the structure of cytoplasmic protrusions which were formed in some of the free EH-118 MG but not in the 118 MG cells. Their nature is completely unknown.

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