We report here a five-step purification procedure that led to the isolation from fetal calf bone marrow extract of a tetrapeptide, Ac-Ser-Asp-Lys-Pro (Mr 487), exerting a high inhibitory activity on the proliferation of hematopoietic pluripotent stem cells [defined here as spleen colony-forming units (CFU-S)]. The structure of this molecule was established from amino acid analysis, fast atom bombardment mass spectrometry, and 1H nuclear magnetic resonance spectral data. This structure was confirmed by comparison with the corresponding synthetic molecule, which presents identical physiochemical characteristics and biological properties. Natural and synthetic peptides administered to mice (at a dose of 100 ng per mouse) after one injection of cytosine arabinonucleoside prevent CFU-S recruitment into DNA synthesis.