Currently, there is increased interest in the use of human viral antibodies for prophylaxis and therapy because recent advances in molecular immunology have made it possible to generate large numbers of human monoclonal antibodies of desired specificity and functional activity in short order. The strategy, developed particularly at The Scripps Research Institute and at Cambridge University is based on antigen selection of such antibodies from combinatorial libraries that express Fabs on the surface of filamentous DNA bacteriophage. Fabs or their whole immunoglobulin derivatives that exhibit high avidity for the selecting antigen and high neutralizing activity for the corresponding virus have been identified, and many of these human monoclonal antibodies should prove to be useful in prophylaxis or therapy of presently uncontrolled, medically important human viral diseases.