Healing powers have been imputed as being a feature of the gel from the aloe vera plant for centuries. The recent isolation of the active ingredient, acemannan, has made testing of this drug important. Since the drug appears to enhance monocyte function in other experiments, these studies were designed to test the capacity of acemannan to enhance immune response to alloantigen and to test whether the potential enhancement is a monocyte driven phenomenon. Acemannan did not enhance lymphocyte response to syngeneic antigens in the mixed lymphocyte culture (MLC) but importantly increased alloantigenic response in a dose-response fashion (2.6 x 10(-7) - 2.6 x 10(-9)M). This effect of acemannan was shown to be a specific response and to concur with concentrations of in vitro acemannan achievable in vivo. A separate series of mixing experiments demonstrated that acemannan incubation with monocytes permitted monocyte driven signals to enhance T-cell response to lectin. It is concluded that acemannan, the active ingredient of the aloe vera plant, is an important immunoenhancer in that it increases lymphocyte response to alloantigen. It is suggested that the mechanism involves enhancement of monocyte release of IL-I under the aegis of alloantigen. This mechanism may explain in part the recently observed capacity of acemannan to abrogate viral infections in animal and man.