Administration of an extract from the powdered root of the plant Withania somnifera was found to stimulate immunological activity in Babl/c mice. Treatment with five doses of Withania root extract (20 mg/dose/animal; i.p.) was found to enhance the total WBC count (17125 cells/mm(3)) on 10th day. Bone marrow cellularity (27x10(6) cells/femur) as well as alpha-esterase positive cell number (1800/4000 cells) also increased significantly (P<0.001) after the administration of Withania extract. Treatment with Withania extract along with the antigen (SRBC) produced an enhancement in the circulating antibody titre and the number of plaque forming cells (PFC) in the spleen. Maximum number of PFC (985 PFC/10(6) spleen cells) was obtained on the fourth day. Withania extract inhibited delayed type hypersentivity reaction in mice (Mantoux test). Administration of Withania extract also showed an enhancement in phagocytic activity of peritoneal macrophages (76.5 pigmented cells/200) when compared to control (31.5/200 cells) in mice. These results confirm the immunomodulatory activity of W. somnifera extract, which is a known immunomodulator in indigenous medicine.