BACKGROUND The so-called diffusely infiltrated lymphoid tissue of the chicken bursa of Fabricius was previously described as a T-dependent bursal area. METHODS We have analyzed immunohistologically its postnatal development by using a battery of mAbs, most of them raised specifically to chicken T cells, in order to characterize phenotypically the nature of its cell components, appearance, development, and possible functional significance. RESULTS Our results demonstrate that this tissue represents poorly developed lymphoid aggregates, the cell content reminiscent of that found in other lymphoid tissues occurring throughout the avian gut. The first lymphoid elements appear in this bursal area only after hatching, growing rapidly to reach the adult condition in the second week of postnatal life. They consist mainly of T lymphocytes, including principally. CD4+ TcR alpha V beta 1+ cells, which form cell groups and CD8+ cells, and TcR gamma delta+ lymphocytes, which predominate in the subepithelial area and within the epithelium. MHC Class II molecule-expressing cells and IgM-, IgG-, and IgA-positive B lymphocytes also occupy the subepithelial region. CONCLUSIONS We propose that the bursal diffusely infiltrated lymphoid tissue of the chicken represents gut-associated lymphoid tissue involved in mounting immune responses to antigens that reach the bursal lumen via the cloaca.