This present work was undertaken to answer two basic questions (a) is C. argentea lectin part of the general defensive strategy of the plant against predation by animals? (b) if so, how does it act on them? To achieve these goals the lectin from C. argentea seeds was purified to homogeneity and included at a 2% level in a diet containing 10% total protein and given to growing rats for 10 days. In vivo it was noted that the lectin from C. argentea is resistant to gut proteolysis, binds to the cells lining the small intestine and induces enlargement in the small intestine, caecum and colon, kidneys and pancreas compared to control rats exposed to the egg-white diet (EW). As the diet containing the purified C. argentea lectin has the same basic composition and protein content of EW diet, the small intestine, kidney and pancreas enlargements are clearly lectin-specific effects. Moreover the animals exposed to the lectin-containing diet presented a significant reduction in the growth rate and lower values of digestibility, NPU and biological value compared to animals fed on a control lectin-free diet. Thus the data from this present study and the report that the C. argentea lectin has insecticidal activity upon Callosobruchus maculatus larvae which attacks cowpea (Vigna unguiculata) seeds reinforce the hypothesis that lectins take part in the mechanisms against herbivory.