Administration of an extract from the root of the plant Withania somnifera (20mg/dose/animal i.p) was found to inhibit the 20-methylcholanthrene induced sarcoma development in mice and increase the life span of tumour bearing animals. Administration of Withania could inhibit the lipid peroxide formation (152 nanomoles/mg protein) (P<0.01) compared with control (198 nanomoles/mg protein). Withania could increase the GSH level (7.7 micromoles/mg protein) which was lowered in control tumour bearing animals (3.96 micromoles/mg protein). GST level was also significantly increased (451 micromoles/min/mg protein) (P<0.001) in Withania treated animals compared to control animals (205 micromoles/min/mg protein). All the animals in the control group developed sarcoma by the 80th day of carcinogen administration. Only 3 animals in the Withania treated group developed sarcoma by the 105th day. In control animals the survival rate was 40% but in the Withania treated group the survival rate was 100% after 15 weeks of carcinogen treatment. These results indicate that Withania could inhibit 20-methylcholanthrene induced sarcoma development in mice.