Effects of four Si-Wu-Tang (SWT)'s constituents, fructose (Fru), paeoniflorin (Pae), ferulic acid (FA), tetramethyl pyrazine (TP), and their combination on irradiated mice as model of anaemia were investigated, with the purpose of further understanding the relationship between SWT's constituents and activities. Similarly to SWT, oral administration of Fru, Pae, FA, TP and their combination, to some extent, all showed effects of increasing the number of peripheral leukocyte and increasing four types of progenitor cells in bone marrow, including colony-forming unit-granulocyte-macrophage (CFU-GM), colony-forming unit-mature erythroid (CFU-E), colony-forming unit-immature erythroid (BFU-E) and colony-forming unit-multipotential (CFU-mix). Pae and FA showed significant body weight reducing effect, which were largely abolished when they were combined with Fru and TP. The SWT, Fru and combination significantly increased the thymus index while Pae significantly decreased it. Both SWT and TP significantly increased the spleen index but the combination did not. The results suggested that multiple constituents contribute to the promoting effect of SWT on hematopoiesis. Although being a very common compound in plants, the Fru has a special contribution to SWT's effect, which cannot be neglected. It may be an important active constituent that is responsible for SWT's promoting effect on hematopoiesis and immunity. Another suggestion is that when being combined, some effect of one constituent, sometimes is unexpected side effect, may be abolished by other. This may reflect the advantage of multiple constituent characteristics possessed by most TCMs.