It is shown that AcSDKP a new regulator of the hematopoietic system can he generated from thymosin β4 by a one‐step enzymatic cleavage in vitro and in vivo. AcSDKP and Tβ4 were both detected in bone marrow cells (BMC'). Incubation of [3H]Tβ4 with either intact or lysed BMC led to the formation of [3H]AcSDKP whereas the labelled tetrapeptide was not degraded under these conditions. Model enzymatic degradation of Tβ4 carried out with bacterial enzymes suggests that a mammalian endoproteinase Asp‐N might be involved in the formation of AcSDKP through the specific cleavage of the 4Pro‐5 Asp peptide bond of T β4.