In this study, the prophylactic effect of the main lectin present in Aloe vera leaf pulp extract (Aloctin 1) was assayed against Ehrlich ascites tumours in mice. The lectin administered prophylactically before tumour implantation regressed tumour size, however, this activity was less potent than that of the A. vera leaf pulp extract previously shown in our laboratory. Accordingly, serum sialic acid and tumour necrosis factor alpha (TNF alpha) levels, chosen as tumour markers, were decreased significantly by the prophylactic administration of the lectin. The increase in spleen and thymus weights in the group given only Aloctin 1, could be explained by the immunomodulatory and mitogenic effects of lectins. These findings, along with lymphoid hyperplasia observed in spleen and thymus, suggest that the tumour preventive effect of Aloctin I could be due to its immunomodulatory activity. Copyright (c) 2007 John Wiley & Sons, Ltd.