The aqueous suspension of Withania somnifera root powder was investigated for their in vivo and in vitro immunomodulatory properties. W. somnifera showed potent inhibitory activity towards the complement system, mitogen induced lymphocyte proliferation and delayed-type hypersensitivity reaction. Administration of W. somnifera root powder did not have a significant effect on humoral immune response in rats. Our results report immunosuppressive effect of W. somnifera root powder, thus it could be a candidate for developing as an immunosuppressive drug for the inflammatory diseases. © 2006 Elsevier Inc. All rights reserved.