CELO virus infection in immuno- depressed chickens induced either by cyclophosphamide ( CP) or infectious bursal diaease virus (IBDV) was undertaken by the determination of serum antibodies and by the recovery of viruses in feces. At low level of CP as 6 to 8 mg per day-old chicken,the immunosuppression was transient. In CP treated chickens, serum neutralizing antibodes to CELO virus appeared later and titers lowered 4 to 8 folds than those in control chickens. However, booster immune responses of CP treated chickens to CELO virus was similar to control group. Excretion pattern of CELO virus in feces was also similar in both CP treated and control chickens. Immunosuppressive effect of IBDV was somewhat prolonged. Chickens immuno-depressed by IBDV at day old were not responded to primary exposure of CELO virus immunologically until secondary viral exposure was made. Even in booster immune responses, IBDV infected chickens showed poorer response and titers decreased 8 to 16 fold than those of control chickens, Duration of virus excretion in feces was much extended in IBDV infected chickens as long as 6 weeks after viral exposure, while CELO virus recovered only within 3 weeks in control chickens.