Mobilization of hematopoietic stem and progenitor cells (HPCs) is induced by treatment with granulocyte-colony stimulating factor, chemotherapy, or irradiation. We observed that these treatments are accompanied by a release of chemotactic activity into the blood. This plasma activity is derived from the bone marrow, liver, and spleen and acts on HPCs via the chemokine receptor CXCR4. A human blood peptide library was used to characterize CXCR4-activating compounds. We identified CXCL12[22-88] and N-terminally truncated variants CXCL12[24-88], CXCL12[25-88], CXCL12[27-88], and CXCL12[29-88]. Only CXCL12[22-88] could effectively bind to CXCR4 and induce intracellular calcium flux and chemotactic migration of HPCs. CXCL12[25-88] and CXCL12[27-88] revealed neither agonistic nor antagonistic activities in vitro, whereas CXCL12[29-88] inhibited CXCL12[22-88]-induced chemotactic migration. Since binding to glycosaminoglycans (GAG) modulates the function of CXCL12, binding to heparin was analyzed. Surface plasmon resonance kinetic analysis showed that N-terminal truncation of Arg22-Pro23 increased the dissociation constant KD by one log10 stage ([22-88]: KD: 5.4 ± 2.6 μM; [24-88]: KD: 54 ± 22.4 μM). Further truncation of the N-terminus decreased the KD ([25-88] KD: 30 ± 4.8 μM; [27-88] KD: 23 ± 1.6 μM; [29-88] KD: 19 ± 5.4 μM), indicating increasing competition for heparin binding. Systemic in vivo application of CXCL12[22-88] as well as CXCL12[27-88] or CXCL12[29-88] induced a significant mobilization of HPCs in mice. Our findings indicate that plasma-derived CXCL12 variants may contribute to the regulation of HPC mobilization by modulating the binding of CXCL12[22-88] to GAGs rather than blocking the CXCR4 receptor and, therefore, may have a contributing role in HPC mobilization.