Avian leukosis virus subgroup J has been found to infect many types of chickens with various genetic backgrounds. The ALV-J strain NX0101, which was isolated from broiler breeders in 2001, mainly induces the formation of myeloid cell tumors. However, strain HN10PY01, which was recently isolated from laying hens, mainly induces the formation of myeloid cell tumors and hemangioma. In order to determine the difference in pathogenicity of the 2 strains in broiler chickens, 2 groups of chicken embryos were infected with NA0101 and HN10PY01 separately. A comparison was made of the mortality, oncogenicity, body weights, indexes for immune organs, levels of ALV group-specific antigen p27, and mRNA expression levels of the tumor-related gene, p53, in ALV-J-infected birds and immune organs of theses chickens in response to Newcastle Disease Virus (NDV) and avian influenza virus subtype H9 (AIV-H9) vaccination. The results indicated that strain NX0101 was highly pathogenic in broiler chickens and led to a 30% mortality rate and 45% oncogenicity, compared with the HN10PY01-infected birds. Weight of chickens was also significantly lower after 15 wk (P < 0.05). In addition, the mRNA expression levels of tumor-related p53 in medulla, liver, and lung in broilers infected with strain NX0101 were significantly higher than those infected with strain HN10PY01 (P < 0.05). These results indicated that strain NX0101 had a higher replication ability in broiler chickens. The findings of this study will contribute to further elucidating the mechanisms underlying host susceptibility and tumor classification in ALV-J-infected chickens.