Monoclonal antibodies are the secreted products of a hybridoma cell line that is created from the fusion of an antibody-producing lymphocyte and a histocompatible myeloma cell. The hybrid cell inherits the ability to survive in tissue culture from the parent myeloma cell and the capacity to produce antibody from the parent lymphocyte. Each hybrid cell can develop into a clone of identical cells producing identical antibodies that bind to one epitope. Production of such monoclonal cell lines from the fusion mixture requires several rounds of dilution and selection on the basis of specific antibody secretion. Once established, the pure cell line can be stored frozen and recovered to produce unlimited quantities of identical antibodies as required. The vast majority of monoclonal antibodies are derived from mouse cells, although it is also possible to produce them from rat cells. There are a few human monoclonal antibodies derived from hybridomas, but these tend to be very difficult to establish and maintain. All the steps required to make mouse monoclonal antibodies are described here.