MITOGENS induce blastogenesis and mitosis in lymphocytes after reacting with specifie receptors on the cell surface. In contrast to antigen receptors, the receptors for mitogens may be common to large fractions of the lymphocyte population. For example, the lectin concanavalin A (con A) causes mitosis in most T cells. The specificity of the receptors is believed to depend on constituent carbohydrate residues, and the effect of mitogens can be modified strongly by enzymes which govern the removal of carbohydrates from the cell surface1. Thus these enzymes resemble mitogens in that there are specific reacting sites for them on the cell surface. We report here an enzyme which seems to be mitogenic in itself: α-mannosidase produces blastogenesis and mitosis in lymphocyte cultures with an efficiency comparable with that of con A. As α-mannosidase is found in various tissues and cells, including lymphocytes, as well as in blood plasma2–4, our observation may have interesting immunological implications. Another enzyme, galactose oxidase, has been reported to be mitogenic although less strongly than our enzyme5.