Protection experiments using isolated Marek's disease (MD) antigens were performed in antibody-free Rhode Island Red chickens. In addition to virus A antigen (MDV-A) and herpes virus of turkeys (HVT)-induced intracellular (HVT-VIA) antigens, tumour-specific antigens from the lympho-blastoid MSB-1 cells were prepared and isolated plasma membranes were used as vaccines. Neither the tested MDV-A and HVT-VIA nor tumour-specific Marek's disease antigens induced statistically significant protection in vaccinated chickens comparable with that observed after immunisation with live HVT or isolated plasma membranes from infected cells. The specific mortality in the vaccinated chickens was found to be 92% for MDV-A, 76% for HVT-VIA, and 72% for tumour-specific antigens. The result suggested that vaccination immunity against MD is mediated by virus-specific, rather than tumour-specific, antigens.