In the preantibiotic era, passive antibody administration (serum therapy) was useful for the treatment of many infectious diseases. The introduction of antimicrobial chemotherapy in the 1940s led to the rapid abandonment of many forms of passive antibody therapy. Chemotherapy was more effective and less toxic than antibody therapy. In this last decade of the 20th century the efficacy of antimicrobial chemotherapy is diminishing because of the rapidly escalating number of immunocompromised individuals, the emergence of new pathogens, the reemergence of old pathogens, and widespread development of resistance to antimicrobial drugs. This diminishment in the effectiveness of chemotherapy has been paralleled by advances in monoclonal antibody technology that have made feasible the generation of human antibodies. This combination of factors makes passive antibody therapy an option worthy of serious consideration. We propose that for every pathogen there exists an antibody that will modify the infection to the benefit of the host. Such antibodies are potential antimicrobial agents. Antibody-based therapies have significant advantages and disadvantages relative to standard chemotherapy. The reintroduction of antibody-based therapy would require major changes in the practices of infectious disease specialists.