The comparative degradation of N-Ac-Ser-Asp-Lys-Pro (AcSDKP), a negative regulator controlling the proliferation of the hematopoietic pluripotent stem cell, was investigated following incubation with plasma, bone marrow and spleen cells from normal mice and mice bearing a transplantable myeloid leukemia. Using the tetrapeptide, specifically radiolabelled in the lysyl residue, degradation of [3H]AcSDKP was followed by measurement of [3H]Lys formation resulting from its catabolism. It was shown that already after 1 h the degradation of AcSDKP in plasma from leukemic mice was higher compared to that following incubation in plasma from normal mice, whereas incubation with bone marrow cells exhibits a small difference only after 4 hours incubation. However, no increase of AcSDKP catabolic activity was observed following incubation with spleen cells from leukemic animals when compared with incubation of normal spleen cells.