The mechanism of action of the haemoregulatory tetrapeptide Acetyl‐N‐Ser‐Asp‐Lys‐Pro (AcSDKP, Mr = 487 amu), was investigated using an in vitro assay of a murine high proliferative potential colony‐forming cell (HPP‐CFC) which responds to proliferation regulators of the haematopoietic stem cell population. AcSDKP had no direct inhibitory effect on the number, or the proportion in S phase, of the committed granulocyte‐macrophage progenitor (GM‐CFC), or cycling HPP‐CFC populations. However, AcSDKP blocked the action of a stimulator of haematopoietic stem cell proliferation, preventing the switching of quiescent HPP‐CFC into cell cycle. It would appear that AcSDKP exerts its inhibitory haemoregulatory role indirectly, by preventing stimulator action on haematopoietic stem cells.