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e12551 Background: A grade in breast cancer is the strongest prognostic factor. In the new edition of the AJCC 8th, the tumor grade is one of the criteria that determines patients with breast cancer in the prognostic groups. According to the Nottingham system the mitotic count (MC) mostly determines the grade. Interobserver reproducibility for MC is not as good as for IHC marker phh3. The group of triple-positive breast cancer (TPBC) is characterized by the presence of a crosstalk mechanism between the signaling pathways of ER and HER2, which leads to the development of resistance to therapy. However, according to the 8th AJCC, patients with ER+/HER2+ belong to a more favorable prognostic group. Methods: We selected 284 patients with early BC who received treatment at our centre from 2012 to 2020 and didn’t receive presurgical therapy. The patients were divided according to the surrogate subtype into triple-positive (n = 90) and other subtypes (n = 194). The Ki-67, the number of nuclei of tumor cells stained with anti-phh3 antibody, the cell density of the tumor per 1 mm2 and the number of phh3 + per 1000 tumor cells were assessed. Results: Visually accessed number of mitoses was grade 3 in 30% of tumors in the TPBC group and 29% in the second group. There was no statistically significant difference in the Ki-67 index of proliferative activity in the two studied groups. In the TPBC group the median Ki-67 was 16.8%, in the second group - 18.3% (p = 0.376516), while the groups differed significantly in terms of the phh3 index (the median in the TPBC was 23.7/mm2, the median in the second group - 16.06/mm2, p = 0.024703). The groups also significantly differed in the number of nuclei of tumor cells that directly entered the mitotic phase (5.1/1000 nuclei in the TPBC group versus 3.2/1000 nuclei in the second group, p = 0.003901). Conclusions: Despite similar indices of proliferative activity in the TPBC and non-TPBC groups, mitotic activity and the number of directly dividing tumor cells in the TPBC group are significantly higher than in the rest of the BC population. Considering the criteria for assessing phh3 (MC 1 < 4/mm2, MC 2 2 4 to 7/mm2, MC 3 > 7/mm2), most TPBCs will have a histological malignancy grade of 3, which according to AJCC 8th places them in the group of poor outcome. We suggest that these differences may be due to the crosstalk mechanism. The second conclusion of our study is the fact that Ki-67 is a poor predictor of the MC.