Objective: The aim of the study was to investigate the localization, amount and type of lymphocytic infiltrate, found in different molecular subtypes of breast cancer (BC). Methods: Retrospectively, 100 cases of invasive BC were analyzed and immunohistochemicaly (IHC) stratified in four subtypes (Luminal A and Luminal B-like, HER2-positive, and triple negative (TN). The percentage of stromal areas occupied by tumor-infiltrating lymphocytes (TILs) was assessed for H&E slides. The samples were IHC-stained for CD3, CD4, CD8, CD20 and FoxP3. The immunophenotyped lymphocytes - intratumoural and stromal, were separately counted, semi-quantitatively graded and further analyzed. Results: A total of 10% of all tumors were lymphocyte-predominant BC. Intratumoral and stromal TILs were predominantly CD3+T-lymphocytes. High counts of all subtypes TILs - intratumoral and stromal, were most common for TNBC and HER2-positive BC. In TNBC, the intratumoral CD3+TILs are significantly related to CD8+ (p=0.002) and FoxP3+ phenotype (p=0.010). In HER2 BC, the intratumoral and stromal CD3+ TILs were significantly related to FoxP3+ phenotype (p = 0,035 and p= 0.011, respectively). Conclusion:CD3+T-cell mediated immunity, especially the one related to CD8+ and FoxP3+ lymphocytes was the leading one in antitumor response in BC, and high count of intratumoral and stromal lymphocytes predominated in TN and HER2-positive BC. Key words: Breast cancer, tumor-infiltrating lymphocytes, CD3, CD4, CD8, CD20, FoxP3. Citation Format: Popovska SL, Dimitrova PD, Dineva TB. A study on amount, localization and immune phenotype of tumor-infiltrating lymphocytes in different subtypes of breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-05-20.