Mammals are vaccinated against infectious organisms and polypeptides are screened for utility as vaccines by a complementing set of monoclonal antibodies, the first of which antibodies binds specifically to the site on the organism which itself binds specifically to a receptor on a host cell of the mammal, and the second of which binds specifically to the first. Vaccination is done with the second antibody alone, and screening is done by determining whether the polypeptide binds to the first antibody.