Cyclophosphamide (Cy) is an alkylating agent widely used in cancer chemotherapy. It has a bimodal effect on the immune system, depending on the dose and schedule of administration. We have previously demonstrated that a single low dose of Cy has an antimetastatic effect, achieved through immunomodulation, in lymphoma bearing rats. Such a treatment reduced the splenic production of IL-10, TGF-beta, and NO, restoring the lymphoproliferative capacity. A shift from immunosuppression to immunopotentiation induced by low-dose Cy treatment was mainly mediated by a decrease in IL-10 production. The present study focused on the analysis of the modulation of type-1 cytokine levels by treatment with a single low dose of Cy and the effect these cytokines (IL-2 and IFN-gamma) and IL-10 have on primary tumor and metastatic cell growth. Our results suggest that a single low dose of Cy induces a Th2/Th1 shift in the cytokine profile of lymphoma-bearing rats, which may be responsible for its antimetastatic effect. A direct action of IL-10 as a growth factor and IFN-gamma as a cytotoxic factor on metastatic cells is also shown.