Twelve isolations of Newcastle disease virus were made from 77 clinical samples from chickens from conjunctivitis, respiratory disease, proventriculitis and bursal atrophy. Nine of the isolations were made from chickens with conjunctivitis. The viruses were identified as Newcastle disease virus by inhibition of their haemagglutinins with specific antiserum to Newcastle disease virus. The viruses failed to kill chicken embryos after inoculation into the allantoic cavity and they were judged to be lentogenic strains. There was no evidence that the Newcastle disease viruses were responsible for any of the clinical conditions from which they were isolated. The presence of other agents in 10 of the samples was indicated by reduced production of haemagglutinin in allantoic fluids of infected embryos, by deaths of infected embryos, by the production of cytopathic changes in avian cell cultures and by electron microscopy. Three isolations of infectious bronchitis virus, 2 of avian adenovirus and one of avian reovirus were made. Other samples were suspected of containing infectious bronchitis virus and mycoplasmas, but these were not isolated. The Newcastle disease viruses failed to produce plaques in chicken embryo fibroblast cell cultures and they were separated from the contaminating agents by haemagglutination and elution followed by passage at terminal dilution in chick embryos. No Newcastle disease virus was isolated from 60 caecal tonsils and 60 lung samples from 9-week-old broiler chickens. Eight lung samples yielded mycoplasmas that caused haemadsorption in chicken cell cultures. The mycoplasmas were probably Mycoplasma gallisepticum.