The 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 60th 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 20th and 30th 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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