Three imported broiler parent farms suffering from high incidence of tumor mortality were investigated. Gross pathological lesions were suggestive of the novel subgroup J of avian leukosis virus (ALV) infection. Serological examination of random blood samples for specific antibodies to ALV - J and other viruses confirmed exposure of these farms to infection with ALV - J, reticuloendotheliosis virus and chicken anemia virus. The impact of intercurrent infections, especially with immunosuppressive viruses, was discussed and general control measures were highlighted.