SUMMARY

The relationship of RSVfl(O) to other avian tumour viruses was studied by virus interference and serological cross neutralization. Viruses of sub-group B interfere with RSVfl(O) on chicken cells but not on quail cells. Viruses of sub-groups A and C enhance the focus forming efficiency of RSVfl(O). Antisera to RSV/?(O) and avian tumour viruses of sub-groups A, B and C do not cross-react, but serological cross-reaction was found between RSV,8(O) and sub-group D. The ability of RSV (O) and L-R cells to stimulate anti-viral antibodies has been investigated. No helper virus in RSV/?(O) stocks was demonstrable by interference assays