Chickens could be protected against lethal lymphoblastic leukaemia due to the nonproducer JMV Marek's disease (MD) tumour transplant by infection with the herpesvirus of turkeys (HVT) or various strains of MD virus. However, solid JMV tumours developed in MD virus-infected birds at the site of intramuscular or subcutaneous transplantation, but tumours never developed at the site of MD virus inoculation. The incidence and extent of local tumour growth, the development of metastases and the inhibition of tumour regression were related to the pathogenicity of the MD virus strains used for pre-treatment of the chickens. Infection of chickens with reticulo-endotheliosis virus (REV-C) or with chick syncytial virus (CSV), which are nonprotective against MD virus or JMV transplants, stimulated local tumour development of the attenuated JMV-A variant of the JMV transplant. Chickens which did not reject local tumours died of visceral JMV tumour metastases. A direct helper mechanism of viral infection on the oncogenicity of transplants was excluded. The results suggested that virus-induced immunosuppression stimulated the development of local JMV tumours which never occurred in normal chickens. Immunity to the JMV transplant, including resistance to lethal leukaemia and successful regression of local tumours, did not coincide with immunity to MD virus-induced visceral lymphomas or nerve lesions. Vaccinal induced tumour immunity evidently was defective. The significance of these results is discussed with reference to immunological functions of MD tumour-specific antigens.