The tetrapeptide AcSer-Asp-Lys-Pro (AcSDKP) is a physiological inhibitor of the proliferation of haematopoietic stem cells and progenitors. In Ara-C-treated mice, its plasmatic concentrations decrease while the CFU-S start cycling. Infusion of synthetic AcSDKP (Goralatide) at this time protects them from haematoxicity by blocking early cycling of CFU-S. Both in vitro and in vivo, this effect seems to be optimal in a narrow range of concentrations. Thus, a better knowledge of the kinetics of endogenous AcSDKP during cancer treatment could help to optimize the treatments with Goralatide. AcSDKP plasma levels have been measured by a specific EIA in 14 cancer patients during the two initial monthly 5 day courses of chemotherapy with 5-FU alone administered either by continuous infusions (six patients) or by 1 h daily infusions (eight patients). AcSDKP concentrations did not vary significantly during the first and the second course. Together with our previous results in AML patients treated with high doses chemotherapy (Ara-C and Anthracyclin), our present data suggest that the variations of endogenous AcSDKP in patients are dependent of the type, doses and schedule of chemotherapy.