Exposure to infectious bursal disease virus (IBDV) at 1 day old followed by inclusion body hepatitis virus (IBHV) inoculation at 36 days produced typical lesions of hemorrhagic-aplastic anemia syndrome (HAS). The lesions included severe anemia, widespread hemorrhages and dermatitis. HAS could not be induced in the first 4 wk in chickens inoculated at 1 day old with IBHV alone or in combination with IBDV. The immunosuppressive effects of IBDV failed to alter the pathogenicity of IBHV in chicks less than 4 wk old. This resistance was considered to be age-related. S.c. inoculation of day old chicks with IBDV produced a more severe infection than did oral exposure. Serial passage of IBHV in day old chicks had no significant effect on the viral pathogenicity.