The essential oil from a lemon grass variety of Cymbopogon flexuosus was studied for its in vitro cytotoxicity against twelve human cancer cell lines. The in vivo anticancer activity of the oil was also studied using both solid and ascitic Ehrlich and Sarcoma-180 tumor models in mice. In addition, the morphological changes in tumor cells were studied to ascertain the mechanism of cell death. The in vitro cytotoxicity studies showed dose-dependent effects against various human cancer cell lines. The IC(50) values of oil ranged from 4.2 to 79 microg/ml depending upon the cell line. In 502713 (colon) and IMR-32 (neuroblastoma) cell lines, the oil showed highest cytotoxicity with IC(50) value of 4.2 and 4.7 microg/ml, respectively. Intra-peritoneal administration of the oil significantly inhibited both ascitic and solid forms of Ehrlich and Sarcoma-180 tumors in a dose-dependent manner. The tumor growth inhibition at 200 mg/kg (i.p.) of the oil observed with both ascitic and solid tumor forms of Ehrlich Ascites carcinoma was 97.34 and 57.83 respectively. In case of Sarcoma-180, the growth inhibition at similar dose of oil was 94.07 and 36.97% in ascitic and solid forms respectively. Morphological studies of the oil treated HL-60 cells revealed loss of surface projections, chromatin condensation and apoptosis. The mitochondria showed apparent loss of cristae in the cells undergoing apoptosis. The morphological studies of Sarcoma-180 solid tumor cells from animals treated with the oil revealed condensation and fragmentation of nuclei typical of apoptosis. Morphological studies of ascites cells from animals treated with the oil too revealed the changes typical of apoptosis. Our results indicate that the oil has a promising anticancer activity and causes loss in tumor cell viability by activating the apoptotic process as identified by electron microscopy.