An effective vaccine for Marek's disease may be prepared using a viral agent which is a Marek's disease virus unable to express a functional meq protein. This viral agent is effective to elicit an immune response in a chicken to very virulent strains of Marek's disease virus without causing a significant degree of pathogenicity in the inoculated bird. Suitable formulations of the vaccine for use in chickens include an effective immunization dosage of this novel viral agent with a pharmaceutically acceptable carrier or diluent.