Newcastle disease virus ("Komarov" strain) was propagated in CER monolayer cultures for 40 successive passages. Cytopathic changes (CPE) consisted mainly of rounding of the cells followed by complete destruction of the cell sheet. Upon adaptation, CPE started to appear after 18 hours and complete destruction of the cell sheet occured by the end of 36 hours.
The titer started to increase and become constant by the 36th passage reaching its maximum titer (5 x 10 TCID /ml). It is recommended to harvest the virus between the 20th and 40th hour post-infection, the time at which peak titer is obtained. Fourty five days-old susceptible chickens vaccinated with the 40th passage of this adapted virus, resisted challenge with the virulent virus given 21 days post-vaccination. Immune response in the vaccina ted birds demonstrated high Hl titers of 80-640 and 320-1280 at 15 and 21 days post-vaccination respectively.