Isolation of two transforming viruses from sarcomas obtained in chickens inoculated intraembryonally with a transformation defective mutant of Prague strain Rous sarcoma virus.

Sarcomas appeared after long latency with a frequency of about 2% in Brown Leghorn and (CB X IC)F1 chickens after intraembryonic and neonatal inoculation of transformation-defective mutants of ASVs subgroup C. Only the freshly isolated td mutants, td daPR-C and td daPR-C morphf, exhibited the tumorigenic activity, whereas the standard td mutants induced no sarcomas. Two (862 and 2257) out of four tumours could be transplanted in young chickens and produced low titres of transforming virus. The other two tumours were not transplantable and devoid of any transforming virus. Viruses 862 and 2257 are clearly defective in replication, virus 2257 cannot be complemented efficiently by any helper virus in vitro. The low titre of virus 2257 is not caused by interference with a helper virus of the same subgroup specificity. Both viruses are highly tumorigenic in vivo.