The effect of 75% methanolic extract of the plant Phyllanthus amarus (P. amarus) was studied against cyclophosphamide (CTX) induced toxicity in mice. Administration of CTX (25 mg/kg b.wt, i.p.) for 14 days produced significant myelosuppression as seen from the decreased WBC count and bone marrow cellularity. Administration of P. amarus extract at doses 250 and 750 mg/kg b.wt significantly reduced the myelosuppression and improved the WBC count, bone marrow cellularity as well as the number of maturing monocytes. CTX treatment also reduced the activity of glutathione system and increased the activity of phase I enzyme that metabolize CTX to its toxic side products. P. amarus administration was found to decrease the activity of phase I enzyme. Administration of P. amarus also increased the cellular glutathione (GSH) and glutathione-S-transferase (GST), thereby decreasing the effect of toxic metabolites of CTX on the cells. Administration of P. amarus did not reduce the tumor reducing activity of CTX. In fact, there was a synergistic action of CTX and P. amarus in reducing the solid tumors in mice. Results indicated that administration of P. amarus can significantly reduce the toxic side effects of CTX and is not interfering with the antitumor efficiency of CTX.