A novel generation of rabbit monoclonal antibodies for estrogen (ER) and progesterone (PR) receptor evaluation in breast cancer by immunohistochemistry has been released recently. We compared the novel RabMab anti-ER and anti-PR antibodies with the mouse monoclonal antibodies using a tissue microarray of breast carcinomas. Two cylinders (2mm diameter) of formalin-fixed, paraffin-embedded tissue were obtained from 24 invasive breast cancers and were immunostained using anti-ER mouse (1D5 and 6F11) and rabbit antibodies (SP1 and B644), and anti-PR mouse (PgR312 and PgR636) and rabbit antibodies (SP2 and B645). The immunohistochemistry was evaluated by considering positive those tumors in which more than 10% of the tumor cell nuclei stained independently on the staining intensity. Our results demonstrated that rabbit antibodies against ER have a similar staining pattern compared to the 6F11, but better than 1D5 from three different suppliers. The rabbit antibodies against PR (SP2 and B645) provide a stronger and sharper immunohistochemical signal compared to mouse antibodies (PgR636 and PgR312). Both ER and PR rabbit antibodies allow a lower cost per test because of higher working dilutions compared to mouse antibodies using the same procedure. The novel rabbit antibodies against ER and PR are highly sensitive for immunohistochemical testing of breast carcinomas.