The ontogenic development of the bursa-dependent lymphoid tissue in the chicken has been studied using rabbit antisera specific for B lymphocyte sub-populations and two elements of the bursal microenvironment. The antigens investigated were: the chicken B lymphocyte antigen ( CBLA ); mature B lymphocyte antigen ( CMBLA ); a foetal-associated antigen ( CFAA ) present on embryonic haemopoietic cells and adult bone marrow and bursa cells; immunoglobulin (Ig) and IgG; a bursa-specific cortical reticulin fibre antigen ( CBRFA ); a gut-associated mucin antigen ( CGAMA ) present on bursal medullary reticular epithelial (REp) cells. The development of suspected precursor cells was examined using a rabbit antiserum specific for foetal spleen cells. The major finding was the interrelationship between developing B cells and the bursal microenvironment. The first CBLA - and CFAA -positive cells were detected in the bursa at day 8 of incubation but their precise localization was difficult to assess. In 12-15 day embryos, both these cells were found predominantly in the tunica-propria in close proximity to cells bearing the reticulin fibre antigen CBRFA . This close association between CBLA -, CFAA - and CBRFA determinants represents the earliest stages of B cell differentiation and mimics that found in the adult bursa cortex. By day 18, the majority of bursa cells expressed CBLA and Ig and were localized in the developing medullary follicles, the REp cells of which were CGAMA -positive, demonstrating a very early interaction between intestinal tract contents, bursal REp cells and B cell maturation. Around hatching some bursa cells showed a marked increase in the membrane expression of CBLA and Ig, and the simultaneous expression of CMBLA and IgG. These cells were present in medullary follicles; CBRFA was present on cortical reticulin fibres which provided a supporting framework for the more immature CFAA -positive cells.