Aloctin A (Alo A), an active substance isolated from the leaves of Aloe arborescens Miller, has many biological and pharmacological activities, such as mitogenic activity for lymphocytes, binding of human α2-macroglobulin, complement activation via the alternative pathway, antiinflammatory activity, antiulcer activity and antitumour activity. In this paper, the effects of Alo A treatment in vivo and in vitro on the immune response of murine and human lymphoid cells are described.