Cyclophosphamide (CTX) has been used to precondition recipient hosts before adoptive T cell therapy to tumor. The aim of the present study was to analyze the host cellularity changes resulting from different doses and timing of CTX administration. To address this aim, Ehrlich ascites carcinoma (EAC)-bearing mice (5×10⁵cells/mouse) were intraperitoneal injected with 2 or 4 mg/mouse CTX either on day 1 or 7. The results revealed a significant decrease in the number of the tumor cells at 4 mg/mouse CTX on day 1 with an induction of 99% (11.6×10⁶ cells/µl) death of the EAC cells. On day 1 post-EAC cells inoculation, there was a significant increase in the absolute number of splenocytes in EAC-bearing mice at 2 and 4 mg/mouse CTX. The absolute number of leukocytes, lymphocytes and monocytes in EAC-bearing mice non-significantly increased at 2 mg/mouse CTX on day 7 and at 4 mg/mouse CTX on day 1 and 7. Additionally, the absolute number of neutrophils was significantly increased at 2 mg/mouse CTX on day 7 and non-significantly increased at 4 mg/mouse CTX on day 7. The results indicate that: 1) the anti-tumor effect of CTX is a dose- and time-dependent, 2) the use of high dose of chemotherapy is preferable to anti-tumor adoptive cell therapy since it induces higher anti-tumor effects with less lymphopenia.