The most reliable method of distinguishing strains of low virulence amongst a collection of stock strains of Newcastle disease virus (NDV) was by their failure to produce a good c.p.e. in monolayers of baby hamster kidney cells and of chick embryo cells. This method was of no help in identifying avirulent mutants that emerged in the Essex 70 strain of NDV following ultraviolet light, nitrous acid, hydroxylamine or -methyl--nitro--nitrosoguanidine (NTG) treatment. A marked reduction in the ability to kill developing chick embryos at 41 °C was a much more reliable indicator. Several of these temperature-sensitive mutants, most of which had been isolated from NTG-treated virus, were non-lethal for young chicks but they did have a depressive effect on their growth rate. The immunity produced by three of these mutants in chicks free of NDV antibody, but not in chicks possessing appreciable amounts of antibody, was probably even better than that produced by Hitchner B1 strain. All three mutants reverted to virulence during passage in chicks, although in no case were the revertants as virulent as the original Essex 70 strain. The virulent revertants obtained from one of the mutants had lost their temperature-sensitivity and proliferated in large numbers in the tissues of infected chicks. Those obtained from the other two had either not lost, or only partly lost, their temperature-sensitivity; they were found only in low concentrations in the tissues of infected chicks, their concentrations being little different from that found in the tissues of chicks infected with the mutants from which they were derived.


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