Withania somnifera, commonly called Ashwagandha in the Indian traditional system of medicine has been reported for several pharmacological activities. This study demonstrates, for the first time, the potential role of the chemically standardized leaf extract of W. somnifera (WSL) and it's identified component in activating immune system. WSL enhanced Th1 cytokine IFN-gamma expression in Con A primed splenocytes in vitro. When given orally for 2 weeks to BALB/c mice immunized with emulsion of OVA in Freund's adjuvant (OVA-FCA), it caused dose-dependent proliferation of T cells and improved their ability to secrete IL-2 and IFN-gamma, but moderately down-regulated Th2 cytokine IL-4. Flow cytometric analysis of lymphocyte surface markers of T cells CD3(+), CD4(+) and CD8(+), and B cells CD19(+) indicated prominent enhancement in proliferation and differentiation of lymphocytes. Further, the effect of WSL in immunized mice elicited up-regulation of beta-integrins LFA (CD11a) and Mac-1 (CD11b) in splenocytes. Co-stimulatory molecules CD80 and CD86 that are important secondary signals for the activation of immune system elicited remarkable enhanced expression when observed in spleen-derived macrophages isolated from WSL treated mice. Chemical standardization of WSL suggested that the withanolide 2,3 dihydro-3-sulphonile withanone is a major constituent of WSL responsible for skewing to Th1 immune polarization by stimulating the expression of IFN-gamma and B cell switch over to secrete IgG2a while simultaneously enhancing the expression of co-stimulatory molecules and integrins. These studies demonstrate the possible usefulness of WSL and its major constituent WSL-2 as Th1 immune adjuvants for chronic infectious ailments where patients suffer from weakened Th1 immunity.