Abstract
When an avian sarcoma virus (ASV), subgroup C Bratislava 77 (B77-C) was inoculated into duck embryo fibroblast cultures (DEF) at a m.o.i. of 0.02, its replication was retarded by about 3 days compared with that in chick embryo fibroblast cultures (CEF). A transformation-defective (td) mutant was isolated during this period of retardation. Unlike the sarcoma virus, this td mutant replicated in both DEF and CEF with no retardation, even at a low m.o.i. The subgroup C Prague strain of Rous sarcoma virus (PR-C), which can infect DEF, also replicated in DEF slower than its td mutant, tdPR-C, at a m.o.i. of 0.02.