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In the present study, we aimed to evaluate estrogen receptor ER-alpha status in 61 breast cancer cases using Sp1 and 1D5 monoclonal antibodies. Tissue array platforms were generated containing samples of breast cancer and positive controls that were assayed by immunohistochemistry applying monoclonal primary antibodies anti-ER alpha, SP1 and 1D5. We noted a high concordance rate (96.7%) between the referred antibodies. Moreover, we calculated the Kappa factor (0.921), indicating that 1D5 and SP1 provided overlapping ERα expression results. Indeed, we observed controversial results only in 2 samples studied, which were ER-negative when stained with 1D5 and ER-positive when assessed with SP1. Total concordance of PS was obtained (Pearson and intraclass CF, 0.7351 and 0.6193, respectively). However, concordance between the antibodies seems to be more accurate in higher PS values. An excellent IS correlation between antibodies was observed throughout the population (Spearman's CF, ρ=0.9150). Following the Allred score, 17 out of 42 positive BC samples diverged, with 1D5 always pointing to weaker staining than SP1. When calculating Spearman's CF of Total Score (TS) within the population, an excellent correlation between both the antibodies (ρ=0.9238) was noted. Nonetheless, the results were less concordant among the BC-positive cases (ρ=0.7743). Indeed, 20 samples were differentially classified using the antibodies (only 3 had higher TS with 1D5). Considering the mean TS of all samples or of invasive ductal carcinoma, SP1 provided higher scores than 1D5 (p