The pretreatment of isolated of Langerhans with concanavalin A (Con A) completely blocks alloxan from suppressing the insulin release response to glucose. The lectin itself inhibits insulin secretion. This effect is dose dependent and reversible. Con A, however, has no protective action against the inhibition of glucose-induced insulin biosynthesis in islets exposed to alloxan. The protective action of Con A on alloxan toxicity is likely to be at the beta-cell surface at a membrane recognition site for glucose as a stimulus for secretion. The insulin biosynthetic effect of glucose appears to be mediated through a separate mechanism.