Abstract THE etiologic agent of Marek’s disease (M.D.) has been readily transmitted among chickens by direct and indirect contact (Sevoian et al., 1962, 1963) and may survive in the environment for many weeks (Witter et al., 1968). Initial attempts to propagate the M.D. agent in vitro were unsuccessful, but Churchill and Biggs (1967) and Solomon et al. (1968) were successful in propagating the M.D. agent in cell culture. Churchill and Biggs (1967) and Nazerian et al. (1968) identified the etiologic agent as a cell-associated group B herpesvirus. Eidson et al. (1969), Nazerian and Witter (1970) and Calnek et al. (1970) found that a cell free preparation of the M.D. virus (M.D.V.) is infectious for chickens. Churchill et al. (1969) found that a highly pathogenic strain of M.D.V. became attenuated after passage in cell culture, and when administered to chicks it protected against the development of M.D. tumors. Kawamura et al. (1969)…