Abstract Concanavalin A binds to human leucocytes and can be dissociated by methyl alpha- d-mannoside. It stimulates incorporation by the cells of uridine and thymidine in the presence or absence of plasma. Incorporation of thymidine is proportional to the concentration of concanavalin A up to approx. 10 μg/1.5 million cells. Maximal incorporation of thymidine occurs at about 72 h. The reaction is inhibited by sugars in the order methyl alpha- d-mannoside > methyl alpha- d-glucoside > mannose, consistent with their known affinities for concanavalin A. Inhibition by methyl alpha- d-mannoside is proportional to its concentration. Stimulation by concanavalin A of DNA synthesis is suppressed by addition of methyl alpha- d-mannoside to the culture medium even after 20 h.