The first description of the hybridoma technology in 1975 seemed to hold enormous promise for the treatment of a variety of human disease. The ability to produce monoclonal antibodies led to the availability of large amounts of homogeneous and predictable preparations of antibody. The potential to renew indefinitely a particular antibody surmounted many of the technical and regulatory problems that made polyclonal antisera difficult to use as therapeutic agents in man. The hybridoma technology seemed even more valuable as it became clear that it could lead to the generation of pure, highly specific antibodies from impure, poorly characterized antigens. Monoclonal antibodies have been extremely useful in basic investigations and have facilitated the development of new diagnostic tests for serum and tissue components and infectious agents. However, novel approaches are needed in order to provide more useful, less immunogenic antibodies which could be used routinely for passive immunization in the treatment of infections or for tumor targeting.