Immunostimulatory activity of AC II, a registered ayurvedic preparation prepared at Amala Ayurvedic Research Centre for treating HIV and AIDS is reported. AC II administration could significantly enhance the mitogen-induced proliferation of lymphocytes of spleen cells. It was also found to increase cell-mediated immune responses in normal and tumor-bearing control animals. Oral administration of AC II significantly enhanced Natural Killer cell activity in normal and tumor-bearing animals on the 7th day, which was observed earlier than the tumor-bearing control animals and normal animals. Antibody dependent cellular cytotoxicity (ADCC) was also increased in AC II treated normal and tumor-bearing animals. An early enhancement of antibody-dependent complement-mediated cytotoxicity was also observed by the administration of AC II in normal as well as tumor-bearing animals. Treatment with AC II elevated the levels of IL-2, TNF-alpha and IFN-gamma in normal mice. Administration of AC II was also found to increase the cytotoxic T lymphocyte production in EL4 treated mice. These studies support the use of this immune stimulatory preparation in HIV patients.