There is an increasing interest in the preparation of rat × rat hybridomas, because they have been found to be more stable in culture than mouse hybridomas and they secrete consistently high levels (10 µg/mL and above) of monoclonal antibody. In addition, certain subclasses of rat IgG have been found to interact efficiently with human Fc receptors and the human complement system. More important perhaps in practical terms is that, when grown as an ascites, up to 30 mL of fluid containing 1-5 mg/mL specific antibody can be obtained from each rat. Furthermore, when nude rats are used for ascites production, the levels of endogenous immunoglobulin can be less than 1 mg/mL.