Abstract
Avian sarcoma virus (ASV)‐transformed mammalian cells, known to be either virus‐producing, virogenic or non‐virogenic, were tested for viral gene products: for p27 by radioimmunoassay and for group‐specific (gs) antigens by the complement fixation test. Clones of B77 virus‐producing RBI rat tumour cells had p27 concentrations ranging from 10 to 80 ng/mg cellular protein; the poor virus‐producing clone had the lowest levels of p27 and gs antigen. Treatment of the clones with halogenated pyrimidines, BrdUrd and IdUrd, had little effect. Virogenic mammalian cell lines XC, LWC3 cells and human 118MG/EH cells transformed by the Prague, Schmitt‐Ruppin and Engelbreth‐Holm strains of ASV, respectively, were analyzed for p27, for gs antigens, tumour induction in chickens and focus formation after fusion of the X‐ray‐irradiated cells with chicken fibroblasts. Concentrations of p27 and of gs antigens and tumour induction by intact cells were approximately the same in both rat XC cells and human 118MG/EH cells. Treatment with BrUrd or IdUrd increased the level of p27 in the latter cell line. On the other hand, rat LWV3 cells had no detectable p27 or gs antigens but their tumourinducing activity in chickens was fifty‐fold higher. Non‐virogenic rat TWERC cells, transformed by the Prague strain of RSV, had low detectable levels of p27 and of gs antigens. These cells did not produce tumours in chickens or foci when fused with chicken fibroblasts. The only, and very effective, way of rescuing RSV from TWERC cells was the fusion of TWERC cells with RAV‐1‐preinfected cells. After inoculation of a mixture of 2×10^4^ cells treated with inactivated Sendai virus, tumours were induced in 50% of the chickens. Both the host‐range and virus neutralization studies indicated that the rescued TWERC virus had avian subgroup C specificity and not the specificity of the helper virus. The rescued virus had a one‐hit titration pattern in duck embryo fibroblasts, indicating that the virus was non‐defective for infection and transformation of these cells.