Normal bovine and mouse sera contain a component, termed 13 inhibitor, that inhibits the infectivity and hemagglutinating activity of influenza A viruses of the HI and H3 subtypes. We have previously shown these 13 inhibitors to be mannose-binding lectins that apparently act by binding to carbohydrate on the viral hemagglutinin, blocking access of the receptor-binding site to receptors on host cells (E. M. Anders, C. A.