The stimulatory effect of Withania somnifera on cytotoxic T lymphocytes was determined in Balb/C mice by the Winn's Neutralization assay using CTL sensitive EL4 thymoma cells as target cells. Withania somnifera showed a significant increase in CTL production both in vivo as well as in vitro. The survival time of E14 cells alone in treated animals was only 21 days and in the Withania extract treated group was 45 days with a percentage increase in life span (% ILS) of 114.28%. The survival rate of animals administered with EL4 cells (target) which were incubated with alloimmunized spleen cells (effector) from normal Balb/c mice was 33 days (% ILS 57.14%) and when this group was treated with Withania extract (20 mg/dose/animal i.p. for 10 days) their life span was increased to 53 days (% ILS 152.38%). The survival days of animals after the administration of EL4 cells incubated with alloimmunized spleen cells (effector) from Withania treated animals was 48 days (% ILS 118.18%) and when these animals continued with the Withania treatment their life span increased to 61 days (%ILS 177.27%). There was a significant increase in life span when animals were treated with EL4 cells incubated with effector cells generated by mixed lymphocyte culture of Balb/c mice and C57BL/6 mice in the presence (172.73%) and absence of Withania extract (59.09%). These studies indicate that Withania somnifera could stimulate the generation of cytotoxic T lymphocyte and it may reduce tumour growth.