Proteins of leukemia and sarcoma viruses from the chicken, mouse, hamster, and cat were analyzed by gel filtration in guanidine hydrochloride. The mammalian viruses were found to contain six major proteins, whereas the avian viruses contained seven proteins. Proteins of viruses from different mammalian species had the same molecular weights which also closely resembled the molecular weights of the six equivalent avian viral proteins. These results defined a basic similarity in protein composition for the C-type viruses of avian and mammalian origin. The two glycoproteins of murine leukemia virus were identified immunologically as constituents of the viral membrane. Antisera prepared against other proteins distinguished individual internal viral antigens in immunodiffusion tests and also reacted in immunofluorescence with cytoplasmic components of infected cells. Antisera which reacted with the major internal viral proteins did not contain antibodies inhibitory to the viral reverse transcriptase.'4C-L-amino acid mixture per ml, 5,uCi of 3H-L-amino acid mixture per ml, and 2,uCi of 3H-glucosamine per 359 on July 6, 2020 by guest

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