The development of the hybridoma technology by G. Köhler and C. Milstein in 1975 enables us to produce monoclonal antibodies with desired specificities for the application in many branches of the biological sciences. The following article describes the technique of hybridizing spleen cells from immunized mice with myeloma cells. The resulting hybridoma produce monoclonal antibodies which have been applied in a wide variety of immunoassay methods. They are also useful in the structural analysis and quantification of antigens and have many applications as diagnostic and therapeutic agents for different tumors. There are already several reports of trials in which monoclonal antibodies eliminate neoplastic cells from bone marrow.