The tetrapeptide Acetyl-N-Ser-Asp-Lys-Pro (AcSDKP) interferes with G1/S phase progression in hematopoietic precursors. We investigated the effect of AcSDKP on in vitro and in vivo hematopoiesis in a canine model. AcSDKP, added daily for 2 weeks to long-term marrow culture (LTMC) at concentrations > 10(-8)M, reversibly inhibited colony-forming unit granulocyte/macrophage (CFU-GM) formation (p < 0.001 and p < 0.05 for 10(-6) and 10(-7)M, respectively). Inhibition was more profound when AcSDKP addition was begun at the initiation rather than at the time of recharging the cultures. Next, seven dogs were given AcSDKP in vivo at 50 (n = 2), 250 (n = 2), or 500 micrograms/kg/day (n = 3) via continuous infusion for 7 days. No adverse effects were observed. LTMCs were established on days -9, -2, +7, and +28 of AcSDKP. One week later (days -2, +5, +14, and +35), adherent layers were recharged with fresh autologous marrow, and CFU-GM in nonadherent cells was assayed weekly beginning 1 week after recharging. The cumulative number of CFU-GM harvested from LTMCs was dependent upon the time of initiation of LTMC. The difference between day -2 (adherent layer pre-AcSDKP; recharge on AcSDKP) and day +7 culture (adherent layer on AcSDKP; recharge after discontinuation of AcSDKP, p < 0.001) suggested an effect of AcSDKP on the adherent stromal layer. Ex vivo hematopoiesis partially recovered following discontinuation of AcSDKP, although CFU-GMs were still reduced in LTMCs established on day +28. Normal nonadherent cells recharged onto allogeneic adherent/layers obtained during AcSDKP treatment grew significantly fewer CFU-GM than cultures on adherent cells obtained before AcSDKP treatment (p < 0.05). Therefore, these data suggest that AcSDKP affects not only hematopoietic cells but also cells of the adherent layer.