Experiments were conducted to determine the susceptibility of chicks to Marek's disease and the protective effect of the vaccine prepared from herpesvirus of turkey (HVT) on progeny from dams infected with Marek's disease virus (MDV), or HVT, or both when they were young. Chicks from dams infected with MDV or HVT-MDV were protected from the development of Marek's disease when challenged at 3 days of age. The degree of protection decreased when the time of challenge was delayed; no protection was recognized when chicks from MDV-infected dams were challenged at 10 days of age and chicks from HVT-MDV-infected dams at 21 days of age. When inoculated either with cell-free or cell-associated HVT vaccine, sufficient protection against Marek's disease developed within 10 days in chicks from uninfected or MDV-infected dams. On the contrary, the protective effect of HVT vaccine decreased in chicks from HVT- or HVT-MDV-infected dams when challenge was done at 10 days of age; especially, the effect of cell-free HVT seemed to be inferior to that of cell-associated. The effect was improved in some degree at 21 days of age, so far as the examination for cell-free HVT is concerned. It is considered that the progeny from dams infected with MDV may have maternal immunity against Marek's disease and that the progeny from dams infected with HVT may have immunity against HVT.