Acemannan is the name given to the major carbohydrate fraction obtained from the gel of the Aloe vera leaf. It has been claimed to have several important therapeutic properties including acceleration of wound healing, immune stimulation, anti-cancer and anti-viral effects. However, the biological mechanisms of these activities are unclear. Because of this wide diversity of effects, it is believed that they may be exerted through pluripotent effector cells such as macrophages. The effects of acemannan on the mouse macrophage cell line, RAW 264.7 cells were therefore investigated. It was found that acemannan could stimulate macrophage cytokine production, nitric oxide release, surface molecule expression, and cell morphologic changes. The production of the cytokines IL-6 and TNF-alpha were dependent on the dose of acemannan provided. Nitric oxide production, cell morphologic changes and surface antigen expression were increased in response to stimulation by a mixture of acemannan and IFN-gamma. These results suggest that acemannan may function, at least in part, through macrophage activation.