Antibodies have played an important role in microbiology. In addition to their uses in classifying microorganisms and studying evolutionary relationships, antibodies are useful for detecting the microheterogeneity of proteins that arises from recombination and somatic mutation. However, the usefulness of im­ munological assays has been complicated by the difficulty of repeatedly gener­ ating large amounts of antibodies with restricted reactivity, and the heter­ ogeneity of all antisera obtained by conventional methods. These problems reflect the fact that antisera contain antibodies secreted by many antibody­ producing B cell clones; hence such antibodies are termed polyclonal anti­ bodies. Even if the same antigen preparation and inbred animals are used, B cells recognizing different antigenic determinants are activated in each animal so that each antiserum contains a different spectrum of antibodies. Further­ more, the spectrum of antibodies changes during the course of an immune response, so that an animal initially making useful antibodies may subsequently make less useful ones; the reverse situation may also occur. All investigators using conventional antisera face the problem of first obtain­ ing the desired antibodies and then of having to generate an antiserum with similar properties once a useful antiserum is depleted. A more fundamental problem arises because the polyclonality of antisera results in multiple anti­ bodies against different parts of each antigen as well as antibodies against many different antigens. If the antigenic determinant recognized by one of these many antibodies is lost, its loss is likely to go undetected because the vast majority of antibodies will still bind the antigen. The usefulness of antibodies as research tools was greatly facilitated by the discovery by Kohler & Milstein of a technique for routinely producing mono­ clonal antibodies (59). A monoclonal antibody is secreted by the clonal prog­ eny of a single B lymphocyte that has been immortalized by fusing it to cells from a malignant plasma cell (myeloma) line. Such immortalized B cells, called hybridomas, inherit their immortality from the myeloma parent and the genes encoding a particular antibody from the normal B cell. Hybridomas can be frozen and recovered or injected into mice to produce ascites fluid contain­ ing large amounts of the monoclonal antibody. These properties enable many different investigators to use the same antibody and allow the supply of that antibody to be renewed indefinitely.