Summary The virulent Australian strain of Newcastle disease virus (NDV) multiplied in BHK21 cells grown in Hanks’ medium with lamb scrum. In this tissue culture, infectivity of the virus was titrated and its growth was detected by means of haemadsorption of guinea‐pig crythrocytes. A serum neutralization test, using egg‐adapted NDV, was developed. Analysis of the data obtained over a wide range of virus concentrations and serum dilutions allowed the construction of a standard curve relating 50 per cent point of neutralization to virus concentration. This standard curve was fitted to the observed data as a polynomial regression with tenns up to the third degree. After 43 serial passages which included two (41st and 42nd) at limit dilutions, growth curves were determined for both the egg and tissue culture adapted viruses. Tlie titre reached in the maintenance medium at 60 hours’ incubation of infected tissue cultures was 108 TC ID50/ml. with the adapted virus and 106 TC ID50/ml. with the parent virus. The infectivity titre of the tissue culture adapted virus was the same when determined by haemagglutinin production in chick embryos or haemadsorption of guinea‐pig erythrocytes in tissue culture. This comparison was true also of the parent virus.
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French et al. (1965) studied this question.