Progressive tumor growth is regularly accompanied by changes in the cellular constituents of the immune system. Evidence suggests that soluble factors generated during tumor growth can affect the amount of granulocyte-macrophage progenitors. In vitro colony growth of progenitor cells may be an early indicator of the cellular changes associated with tumor growth. Pluchea quitoc has been previously found to modulate the hematopoietic response during bacterial infection. This study was designed to investigate the effects of P. quitoc on the growth and differentiation of bone marrow granulocyte-macrophage progenitor cells (CFU-GM) in Ehrlich ascites tumor-bearing mice. In contrast to the myelosuppression developed in the tumor-bearing animals, treatment with P. quitoc ethanolic extract (250, 500 or 1000 mg/kg) for 3 consecutive days after tumor challenge reversibly stimulated myelopoiesis, restoring the number of CFU-GM to normal. This same dose-schedule also increased colony formation in normal mice as compared to controls. In addi tion, P. quitoc significantly enhanced survival of tumor-bearing mice. These results suggest an immunoregulatory role for P. quitoc in counteracting the tumor-induced myelopoietic suppression as well as usefulness as adjuvant treatment of cancer.