Effects of ethanol extract of Nigella sativa L. on the growth of Ehrlich ascites tumor (EAT), mitotic index, labeling index and on the life span of tumor bearing mice were studied. The results from treated animals showed a decrease in viable cell count, an increase in the life span of EAT bearing mice and an increase in the glutathione peroxidation of heart tissue. Cytological studies have revealed a reduction in the mitosis and DNA synthesis. Treatment with the N. sativa extract also resulted in improvements in the morphological features of tumor cells, along with a reduction in intracytoplasmic vacuoles, the appearance of cell membrane blebbing and the staining intensity. The results of the present study suggest that the ethanol extract of Nigella sativa seeds can generate antioxidants, possess antitumor activity, and ameliorate and prolong the life span of mice bearing EAT.