Specific interactions between antigen and antibody provided the basis for a variety of applications of monoclonal antibodies of mouse origin. In enzymelinked immunoassays (e.1.is.a.) they are used for quantification and qualification of recombinant DNA-derived proteins and contaminants of proteinaceous nature. In addition, in epitope mapping they are a useful tool in characterization of the protein structure. For purification of proteins, monoclonal antibodies can be used for immunoaffinity chromatography to gain the desired protein in high purity within a short period of development. In tumour-imaging, monoclonal antibodies provide a high sensitivity and selectivity to tumour markers, and therefore, improve the detection of solid tumours and metastasis. In tumour therapy, they are used for drug targeting or act as a cytotoxic agent themselves. Monoclonal antibodies, which specifically interact with cell-adhesion molecules are currently developed as immunomodulating or immunosuppressive drugs. The opportunity of humanization of such monoclonal antibodies of mouse origin or the production of human monoclonal antibodies after immunization in vitro will provide future perspectives for the application of therapeutic monoclonal antibodies in cases where the human anti-mouse antibody (HAMA) response so far does not allow a long-term therapeutic application.