Abstract: This experiment was conducted to investigate the pathogenicity of reticuloendotheliosis virus (REV) on 1 and 8-day-old SPF chickens. SPF chickens were inoculated with 1 000 TCID₅₀·bird ^-1 of REV-C99, a cloned strain. The pathogenicity of REV was evaluated by the measurement of growing rate of chicks and the antibody titers to the Newcastle disease virus(NDV). The results showed that REV challenge at a dose of 1 000 TCID₅₀·bird^-1 resulted in significantly (P<0.05) suppressed HI titers of NDV untill 13-week after vaccination for 3 times. For the chicks challenge with REV at 8-d of age, however, there was no significant difference in NDV titers. The results indicated that REV challenge at 1-d of age would retard the growth of chicks and suppress the humoral immunity. The result suggests that harmful effect of REV infection is agerelated and the control of REV at early age is important.