A tremendous technological advance occurred in 1975 when a method was developed to fuse two cells producing a“hybridoma”which secretes a single clone of antibody, having one immunoglobulin (Ig) class, one structure, one affinity, and one specificity for an antigenic determinant. Because monoclonal antibodies are more precise reagents than conventional antisera they open new doors to diagnosis und therapy of disease, and they are useful tools in research. The pathologist uses monoclonals in immunocyto-chemistry to determine tumor type; the surgeon uses monoclonals for immunosupression in renal transplantation; the immunologist uses monoclonals to decipher cellular and humoral inteructions that could not be appreciated with polyclonal reagents. This review outlines the background of monoclonal antibodies und some of their clinically important uses, both in vitro and in vivo. We also project into the future and describe chimeric antibodies and their possible uses.