Chicks were injected intramuscularly with 7.5 mg cortisone acetate (CA) twice a day at hatch, from hatch to 2 days, from hatch to 4 days and from 7–10 days of age. Cortisone acetate administered between 7 and 10 days of age failed to significantly influence precipitin production in 2 of 3 trials while treatment with CA from hatch to 2 days and from hatch to 4 days significantly suppressed precipitin production in all trials. Bursa histology was similar in all CA groups 12 h after the last injection of CA. With the exception of one trial where birds were injected between 7 and 10 days with CA, all birds experienced bursa regeneration upon withdrawal of CA treatment. The results suggest that neonatal injection of CA influences not only bursa development but also may act directly upon the splee
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