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SUMMARYThe herpes-type virus of Marek's disease showed a gradual increase in the rate of development of cytopathic effect on passage in cell culture. By the 6oth passage macroscopic plaques were produced after 6 days under fluid overlay, but no cell-free virus could be recovered after filtration of culture medium.A loss of pathogenicity for chicks was noticed in the virus after 33 passages in cell culture. Furthermore, an antigenic change occurred between the 2oth and 3oth passage which was characterized by the loss of an antigen which could normally be found in the supernatant medium of cultures infected with low passage virus.The possible origin of the attenuated virus is discussed.

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Canine parvovirus isolated from a case of haemorrhagic enteritis in a breeding kennel in England was passaged and cloned in cultured feline and canine cells. No significant evidence of pathogenicity was found during six serial passages of the modified virus back through young dogs. The attenuated virus was excreted by inoculated animals and spread rapidly to uninoculated animals held in contact. When high titre attenuated virus was given to the six-week-old offspring of a seropositive dam a prompt seroconversion was observed. When the attenuated virus was used as an experimental vaccine in 108 pups in an infected breeding colony a highly significant improvement was obtained in the accumulated morbidity and mortality compared with a parallel group vaccinated with modified live feline panleucopenia virus.