A lectin which we purified from the mucous of Ophidiidae Genypterus blacodes showed a potent mitogenic activity which was comparable to that of concanavalin A (con A). On the other hand, the lectin treated with 2-mercaptoethanol showed an anti-mitogenic activity against con A and lipopolysaccharide (LPS). Lymphocytes were separated into a T cell rich fraction and B cell rich fraction by soybean agglutinin. The mitogenic activity of G. blacodes lectin was examined using these cells. It was suggested that the mitogenic activity of G. blacodes lectin was based on the stimulation of T cells. The lectin was also subjected to both the sperm agglutination and the in vitro fertilization tests in mouse. It was demonstrated that: a) the lectin receptors were ubiquitous on sperm; b) in vitro fertilization of mouse ova was completely blocked by the binding of the lectin to the zona pellucida of the ova.