Chickens were infected with infectious bursal disease (IBD) virus in ovo or at different times posthatching to 6 weeks of age. The B- and T-cell responses in the lymphoid tissues and blood were examined sequentially to 8 weeks of age by using indirect immunofluorescence. The proportion of B-cells was consistently lower in infected birds than in controls, especially in chicks infected as embryos or at 1 day old. The proportion of T-cells increased following these early infections but was slightly lower in spleen and blood of birds infected at 1, 4, and-6 weeks of age. Serum IgM levels dropped significantly after infection, regardless of the time of infection. IgG levels decreased following early infection but increased after infection at 1 week old or more. The results strongly suggest that B-cells are the target for IBD virus infection.