Young White Leghorn chickens fed 2.5 microgram of aflatoxin (Afl) per g of diet from hatching until 4 weeks old and infected with infectious bursal disease virus (IBDV) at 3 weeks old had significantly higher mortality and more severely depressed body weights than chicks with aflatoxicosis or IBD alone. Afl-IBDV chicks also had more extensive gross and microscopic changes characteristic of IBD than did IBDV-chicks. None of the treatments significantly reduced antibody responses to Newcastle disease(ND) and infectious bronchitis vaccines or increased susceptibility to challenge with virulent NDV. In a similar experiment chickens fed Afl from hatching to 7 weeks of age had no marked depression in immune response to ND vaccination.