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The genetic control of resistance to RSV (RAV-50) in chick embryos inoculated via the chorioallantoic membrane route was studied in three breeds of chickens. The relative susceptibility to subgroup D infection was 0.01, 0.02 and 0.003 in broilers, White Leghorns and Bantam×Australorp crosses, respectively. The distribution pattern of putative dr gene frequencies were 0.80, 0.68 and 0.90, respectively, in the same three breeds. Liver tumours resulting from the virus infection were observed in a very few birds about 28 days after hatching.