Mice were treated orally with 20 mg/kg/day of cyclophosphamide in their drinking water. Tumor growth curves of low dose metronomic cyclophosphamide (CTX) treated and control xenografts of (A) SW480, and (B) WM239 cells; Mean (SEM) is plotted. No significant differences in growth rates were observed between control and treated mice for either SW480 or WM239 xenografts. Cancer cell toxicity to CTX was evaluated in vitro using the MTT assay (C). Apoptosis was quantified in tumor sections by TUNEL reaction, and while rare TUNEL positive nuclei were seen in cancer cells (white arrow in D), we did not find any TUNEL positive endothelial cells (black arrows), nor any significant differences in apoptotic cells between control and CTX tumors (E). Scale bar = 25 μm.