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Broiler chickens inoculated with cyctophosphamide showed atrophic bursae and severe immuno-suppression. Infectious bursal disease virus (IBDV) inoculation of the birds at 5 weeks of age caused neither clinical signs nor gross lesions. IBDV was re-isolated from some bursae samples. Cyclophosphamide non-treated (CYNT) chickens inoculated with IBDV showed bursal enlargement followed by atrophy. Examination of sections of the organ showed severe lymphocytic necrosis and depletion. IBDV was re-isolated from the bursa and spleen. The geometric mean titres of the IBDV serum neutralizing antibodies for infected cyclophosphamide treated (CYT) and CYNT chickens were 4.3 and 955.4, respectively. The above observations show that CYT chickens did not develop clinical IBD in the presence of infection and severe immunosuppression. They confirm the hypothesis that a healthy bursa is essential for the development of clinical IBD in chickens within the susceptible age range.