The hemoregulatory dipeptide (pEEDCK)2 was shown to stimulate production of a synergistic activity (SA) in 6- to 8-week-old primary stromal cell cultures. The SA increased colony formation by murine bone marrow cells (approximately 50% above control levels) in cultures stimulated by optimal concentrations of L-cell-derived macrophage colony-stimulating factor (M-CSF). An increased number of granulocyte-macrophage colony-forming cells (GM-CFC) was also observed in long-term bone marrow cell cultures following daily administration of dipeptide for 5 days. The increase in GM-CFC was approximately 90% above control as assessed by colony formation in soft agar and coincided with SA production. It appears that the dipeptide augments the production of myeloid progenitor cells through an indirect mechanism mediated by accessory cells.