Estrogen receptor (ER) tumor's status is critical for breast cancer management. A new rabbit antibody clone, EP1, is now available for ER status determination. The objective was to validate the EP1 antibody clone for its use in breast cancer ER status determination in a clinical setting against the previous standard, SP1. EP1 clone was assessed in 130 consecutive cases, including 50 ER-negative (<1% ER expression), 13 ER-low-positive (1% to 9% ER expression), and 67 ER-positive (≥10% ER expression). Using EP1 versus SP1, positive agreement (sensibility) was 92.5% and negative agreement (specificity) was 100%, leading to an overall agreement of 95.4%. All discordant cases (n=6) were ER-low-positive. SP1 was remeasured in 13 ER-low-positive and in 11 ER-negative cases. Overall agreement between SP1 initial tumor status and reassessment was 70.8% in those negative and low-positive cases. In conclusion, EP1 antibody has been validated for use in breast cancer with a positive agreement ≥90% and a negative agreement ≥95%, as recommended. Also, overall agreement between EP1 and SP1 was as good as between the SP1 initial status and SP1 reassessment.