During B-cell development in the avian bursa of Fabricius most of the developing B cells die by apoptosis and only a minority survive to emigrate into the periphery. Recently, it has been shown that when developing bursal cells become mature and ready to migrate they start to express chL12 antigen. The expression of this cell-surface molecule was found to be associated with the survival of the bursal cells both after in vitro culture and after in vivo cyclophosphamide (CY) treatment. The frequency of early apoptotic cells in freshly isolated bursal cells was found to be high. The high susceptibility of these cells to apoptosis is in line with the finding of low bcl-2 mRNA expression. We conclude that expression of avian chL12 antigen is associated with the survival of bursal cells.