The efficacy of chemotherapy is thought to be direct killing of tumor cells, but documented studies have been shown that immunity plays a role in its effectiveness. In a pilot study to observe the bone marrow suppression and regeneration in tumor bearing mice induced by single dose injection of 5-fluorouracil (5-FU), we unexpectedly found that tumors grew fast as bone marrow mononuclear cells (BMC) and peripheral white blood cells (PWBC) were decreased quickly during myelosuppression meanwhile

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... ignificantly slow as repopulating of BMC and PWBC during bone marrow regeneration after 5-FU treatment, no matter whether in low or high dose administration, but the higher the dose was, the lower of the nadir of BMC and PWBC were reached to, as well as the much more powerful duration and strength of the repopulated BMC and PWBC, suggested that the immunity might be a predominant drive in 5-FU chemotherapy. Due to the fact that BMC is the source of PWBC which is its final maturational and functional form, it could be proposed that the anti-proliferative effect of 5-FU on tumor is highly associated with the renewal of PWBC.