Abstract: Previous reports from our laboratory have demonstrated that an reverse phase HPLC (rpHPLC) fraction obtained from extracts of the chicken bursa of Fabricius posses both in vitro anti-steroidogenic activity on avian and mammalian cells and suppression on mitogen stimulated DNA-synthesis in chicken BF cells. Utilizing YM cut-off membranes the bioactive fraction appears to be between ~3-5kDa. However, the identity of such peptide (s) remains unknown. Here, subjected those peptides for mass spectrometric (MS) nano-electrospray quadruple time-of-flight (Q-TOF) MS/MS analysis in an effort to elucidate the composition of predominant fragments. The results of these analyses indicate the presence of small fragments of the non-histone chromosomal protein high mobility group (HMG), nucleophosmin, elongation factor 1-alpha, thymosin β 4 (T β 4), thymosin β, stathmin and histone H1.10. These results indicate that the suppression present in the rpHPLC fraction obtained from the BF, rather than been recognized extracellular messengers, like hormones or cytokines, contains intracellular molecules.