Abstract
The relationship of RSVβ(O) to other avian tumour viruses was studied by virus interference and serological cross neutralization. Viruses of subgroup B interfere with RSVβ(O) on chicken cells but not on quail cells. Viruses of sub-groups A and C enhance the focus forming efficiency of RSVβ(O). Antisera to RSVβ(O) and avian tumour viruses of sub-group A, B and C do not cross-react, but serological cross-reaction was found between RSVβ(O) and sub-group D.