Mouse primary antibodies on rat tissues are advantageous because mouse secondary detection systems can be used with minimum cross-reactivity to endogenous rat IgG. Biocare Medical has developed a Mouse-on-Rat horseradish peroxidase (HRP) polymer detection system with minimal cross-reactivity to rat tissues. It can be used with formalin-fixed paraffin-embedded tissues, floating sections, frozen sections and cell culture preparations. Biocare Medical has novel products to help eliminate persistent rat IgG. XR Factor is a very potent blocker for eliminating endogenous rat IgG. By adding 1-2 drops (1 drop equals 33 µl) of the XR Factor to 2.5 ml of the Mouse-on-Rat Polymer, endogenous rat IgG will be dramatically reduced and/or completely eliminated.