High affinity monoclonal antibodies for recognizing estrogen receptor (clone SP1) with immunohistochemistry and methods for creating such an antibody are disclosed. The lagomorph derived ER antibody provides a significant advantage over the currently available mouse ER antibodies in that there is no need for target retrieval when performing immunohistochemistry. Furthermore, the very low background when the lagomorph derived ER antibody is used in immunohistochemistry is also impressive. The immunohistochemistry comparative study with about fifty clinical specimens showed that the new ER (clone SP1) antibody had favorable results when compared to mouse monoclonal ER antibodies (clone 1D5). The lagomorph derived ER antibody may prove of great value in the assessment of ER status in human breast cancer. Humanized versions of the ER antibody may also provide therapeutic benefits.