We have established two monoclonal antibodies against B-L antigens (chicken Ia-like antigens). The specificity of the antibodies for B-L antigens was determined by two criteria, the cellular expression and the molecular structure of antigens with which they reacted. They reacted with antigens expressed on bursacytes, Con A-blast thymocytes, macrophages, and MDCC MSB1, but not with thymocytes and erythrocytes. In molecular basis, they recognized 64,000 dalton glycoprotein consisting of two polypeptides, 35,000 and 32,000 dalton, which bound non-covalently. To investigate the distribution of B-L antigens on non-lymphoid cells of the bursa of Fabricius, which were thought to play important roles in the differentiation of B cells, anti-B-L antigen and anti-chicken immunoglobulin (Ig) monoclonal antibodies were used. B-L antigen-positive cells were detected in both cortical and medullary areas, whereas Ig-positive lymphoid cells were confined to the medullary areas of normal chicken bursal follicles. In the bursal follicles of cyclophosphamide (CY)-treated chickens, lymphoid cells were depleted but epithelial cells remained intact. And B-L antigen-positive but Ig-negative cells were easily detected in the medullary areas of almost all follicles. These cells were identified to be reticular epithelial cells (REp cells) from the result of their keratin expression.