Isolates of avian sarcoma virus Bratislava 77 (B77) oncogenic for mammals do not contain an excess of transformation-defective mutants.

: Several isolates of the avian sarcoma virus Bratislava 77 (B77), used in tumor induction in rats, hamsters and mice, were tested for the excess of spontaneously segregated transformation-defective mutants (tdB77). The question was asked whether these td mutants could interfere with transforming sarcoma viruses at tumor induction in mammals. It was found that the B77 virus isolates used in successful sarcoma induction in mammals did not contain an excess of td mutants. One virus isolate which had an excess of td mutants did not induce tumors in mammals. The further characterization of the rescued viruses from virogenic mammalian cells showed that all rescued viruses had the same sub-group C specificity as the original isolate of B77 virus. The integration of the viral genome into mammalian cellular genome did not alter transforming ability of the rescued viruses on duck embryo cells. It seems that propagation of B77 virus in conditions in vivo did not support the segregation, and accumulation of an large excess of td mutants in stocks of B77 virus.