Abstract
Upon cultivation of chicken lymphoid tumors induced by Georgia isolate of Marek's disease, clusters of refractile rounded cells appeared consistently in the cell cultures. Intranuclear mature, immature, enveloped, and empty herpesvirus-type particles, usually associated with smaller particles, were observed in all rounded cells in the affected areas. Cytoplasmic inclusions containing mature enveloped virus particles were also seen in the rounded cells. Often fibroblasts growing in the vicinity of the cytopathic effect area contained a few herpesvirus-type particles. The original tumors prior to cultivation did not reveal any virus particles, and the virus in infected cultures was always cell-associated. Control cell cultures neither developed the rounded cells nor demonstrated the presence of any type of virus particles.