Abstract
A line of White Leghorn fowl (line 151) was found to be segregating for susceptibility and resistance to RSV(RAV-O), an avian tumour virus belonging to subgroup E. Family mating within the line, and test matings with other inbred lines, were made to determine the mode of inheritance of susceptibility. The results were fully consistent with the two loci hypothesis advanced previously for control of response to RSV(RAV-O), and not with a single locus hypothesis.