Nevertheless, the concept is important and it should be explored further, especially given our lack of proven interventions to prevent or treat illness due to H5N1 influenza. The use of serum from recovered patients as the source of antibody for passive immunotherapy has the advantage of being technically simple, and ample numbers of convalescing patients should be available for plasmapheresis. The resulting antibody would be polyclonal, which would decrease the chance of an escape mutant developing in treated patients. The serum also might have antibody to other bacterial pathogens, which might decrease the severity of coexisting bacterial superinfections (a mechanism that may account for some of the efficacy of serotherapy in 1918). Balanced against these optimistic considerations are several major concerns. Formidable logistical hurdles would complicate the ability to obtain, characterize, and prepare these materials for use in the midst of an outbreak. As yet, we don't know whether patients who recover from H5N1 influenza develop particularly high levels of antibody (8). Other types of antibody preparations might be more effective, such as pools of serum with high titers of antibody generated from individuals who had received vaccines. With the recent advances in antibody technology, rapid production of humanized monoclonal antibodies with neutralizing activity is possible (9). We lack sufficient understanding of the immune response to H5N1 infection in humans, as well as the potentially protective humoral and cellular responses associated with recovery from disease. We don't know the level of antibody that needs to be achieved to confer protection or the appropriate dose of serum needed to achieve useful antibody levels in recipients. The evidence supporting serotherapy in humans appears to be limited to the experience in 1918.