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Introduction The proliferation factor Ki67 is prognostic in breast cancer and included in international therapy guidelines. In fact, a useful surrogate definition of Luminal A-like as distinct from Luminal B-like disease could be made using a combination of ER, PgR and Ki-67, without requiring molecular diagnostics. Furthermore, Ki67 is a predictive tool for hormonotherapy response in the neoadjuvant setting. Objective To compare two monoclonal antibodies MIBland SP6 for assessment of Ki67 in primary breast cancer with regard to reproducibility. Methods We used TMAs constructed from 82 archival FFPE blocks of primary breast. Staining was performed using Ventana Benchmark Autostainer and MIB-1 antibody (Novocastra) was done at a dilution of 1:100, whereas 1:100 dilution was used for SP6 antibody (Cellmarque). Results Tumor characteristics: ductal carcinomas GH1, 19 cases; GH2, 32 cases; GH3, 21 cases; and lobular 10 cases (classic, 8 and pleomorphic, 2). Criterion standard (Ki67-count) was obtained by counting positive and negative tumor cell profiles. Proliferation index between the two antibodies was compared by 3 independent observers. Proliferation index results were similar from both antibodies and ranged from 3% to 85%. Conclusion Both antibodies can be safely used to determine Ki67 proliferation index. SP6 and MIB1 provide highly comparable measures of Ki67 that predict prognostic in breast cancer.