Effect of Withania somnifera on the cellular immune responses (CMI) was studied in normal as well as tumour bearing animals. Administration of Withania extract was found to enhance the proliferation of lymphocytes, bone marrow cells and thymocytes in responses to mitogens. Both PHA and Con A mitogens along with Withania treated splenocytes, bone marrow cells and thymocytes could stimulate proliferation twice greater than the normal. Withania treated splenocytes along with the mitogen LPS (10 microg/ml) could stimulate the lymphocyte proliferation six times more than the normal. Natural killer cell activity (NK) was found to be enhanced significantly in both the normal and the tumour bearing group and it was found to be earlier than the control (48.92% cell lysis). Antibody dependent cellular cytotoxicity (ADCC) was found to be enhanced in the Withania treated group on the 9th day (65% cell lysis). An early Antibody dependent complement mediated cytotoxicity (ACC) was observed in the Withania treated group on day 13 (47% cell lysis).