Calprotectin, a calcium-bound protein complex, is abundant in the cytosol of neutrophils. It has been reported that this protein has an apoptotic activity in tumor cells. Since calprotectin increases in colorectal cancer, this study was conducted to investigate, for the first time, the cytotoxicity/apoptotic effect of calprotectin on HT29/219 and SW742 colon carcinoma and adenocarcinoma cell lines. MTT (3-(4,5-Dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide) assay, annexin V/PI and Hoechst 33258 staining were used to detect apoptotic cells. The activation of caspase-3 and -9 was assayed using caspase assay kits. Flow cytometer was used to determine if there is any binding of calprotectin to its receptor. Treatment of the cells with different concentrations of human calprotectin resulted in a significant increase in the cell death. Annexin V/PI and Hoechst 33258 staining revealed that the cell death was mainly of apoptotic type. A significant (p<0.05) increase in the activity of caspase-3, and -9 was observed in both cell lines following the treatment. However no binding activity was observed. In Conclusion, this study showed that human calprotectin has an apoptotic effect on HT29/219 and SW742 cells, and that the mode of action is caspase activation particularly via mitochondrial route.