The purpose of the present investigation was to extend previous research conducted by our laboratory which demonstrated the suppressive effects of chicken bursa of Fabricius-derived bursal anti-steroidogenic peptide (BASP) on neonatal-chick B-lymphocyte mitogenesis. To test these anti-proliferative effects on lymphocytes derived from older animals, and to evaluate the phylogenetic applicability of these findings to other species, we evaluated the ability of BASP to alter proliferative indices in peripheral blood lymphocytes derived from adult chickens, rats and humans. Data from the present series of experiments confirm and extend our previous findings and suggest an anti-proliferative effect, exerted by BASP, on pools of isolated PBL from chickens, rats and humans, when measuring 3H-thymidine incorporation following mitogenic stimulation. While the present data was derived from limited numbers of pooled PBL, these findings may be indicative of a potential extra-bursal function of BASP on lymphocyte function.