Bursin is a tripeptide found in follicular and dendritic reticular epithelial cells of the avian bursa of Fabricius that selectively induces the differentiation of committed B-lymphocyte precursor cells but not of committed T-lymphocyte precursor cells. In immunoassays with tissue extracts, bursin is also present in avian and bovine bone marrow. The bursin-containing cells of the intrahepatic bile ducts are not associated with developing B cells and it would appear that mammals have evolved a local hepatic function for bursin.