Abstract Bursal anti-steroidogenic peptide (BASP), purified from the chicken bursa of Fabricius (BF), has been previously demonstrated to be a potent and efficacious inhibitor of steroid hormone biosynthesis from chicken ovarian, and both mammalian and avian adrenal cells in vitro. Other studies have demonstrated that BASP can markedly reduce avian and mammalian mitogen-stimulated lymphocyte proliferation. Recent studies have indicated that BASP has a structural and functional relationship with histone H1. Immunohistochemical studies using a monoclonal antibody, which is known to recognize a common histone H1 epitope from several plant and animal species identified the protein within the cytoplasm and nucleus of distinct cells within both the cortex and medulla of all BF follicles. Additionally, epithelial cells within the BF expressed the protein strongly in the cytoplasm with reduced nuclear staining. In contrast, the same antibody did not recognize the protein in thymus of the same animals. The differential expression of histone H1 immunoreactivity within selected cells of the BF may support a previous proposed role of histone H1 in extranuclear and extracellular signaling in chickens and possibly other species.