A homologue of interleukin-8, viral interleukin-8 (vIL-8) has been identified in the genome of Marek's disease virus (MDV). This protein attracts peripheral blood mononuclear cells in vitro although its role in the pathogenesis of Marek's disease (MD) is not known. P chickens, genetically susceptible to MD, and N chickens, genetically resistant to the disease, were inoculated with either RB1B MDV or RB1BvIL-8smGFP, a vIL8 knockout RB1B MDV, to assess the role of vIL8 in the pathogenesis of MD. The tumor incidence was highest in the P birds given the RB1B virus, where the incidence was 100%. Tumor incidence in N birds given RB1B was 41.5%. Thirty-one percent of the P birds given RB1BvIL-8smGFP developed tumors, and no N bird given RB1BvIL-8smGFP developed tumors. Histologically, the tumors from RB1B-inoculated birds were larger and more invasive and had a more homogeneous cellular composition than those from RB1BvIL-8smGFP–inoculated birds, which were best described as microtumors. These microtumors did not obliterate the normal architecture of the tissues, and in contrast to the RB1B tumors, moderate numbers of heterophils were admixed with the proliferating lymphocytes. Susceptible birds receiving RB1B had the highest viral titers throughout the study, followed by the resistant birds inoculated with RB1B. P and N birds receiving RB1BvIL-8smGFP virus had consistently lower levels of viremia than their RB1B-inoculated counterparts although virus could be recovered from the birds during all stages of MD. In addition, the RB1BvIL-8smGFP virus was detected in birds held in contact with the inoculated group, although no tumors developed in contact control birds. This result indicates that RB1BvIL-8smGFP replicates in vivo but not as well as RB1B and that vIL8 is not essential for the completion of the pathogenesis of MD.