Avian leukosis is a widely distributed disease caused by Avian leukosis virus (ALV). ALV p27 is a capsid protein and group specific antigen, whose role in host immune response is poorly understood. To explore ALV p27, we developed a RAW264.7 macrophage cell line stably expressing p27-GFP fusion protein. The cells of this line and control cell line expressing GFP protein only were compared in production of tumor necrosis factor α (TNF-α), interleukins IL-1β, IL-6, IL-12, and in proliferative activity stimulated by the lipopolysaccharide (LPS). It was found that the ALV p27 expression markedly reduced the production of TNF-α, but did not affect the production of IL-1β, IL-6, and 12, and cell proliferative activity. These results demonstrate that ALV p27 specifically inhibits TNF-α expression in the macrophages stimulated by LPS, suggesting a possible contribution of ALV p27 to immunosuppression detected in ALV-infected hosts.