The aim of this study: The aim of this study was to determine whether the newly synthesized phospholipase C inhibitor from the group of thieno[2,3-b]pyridine inhibitors reduces the percentage of triple negative breast cancer stem cells (CSC) on the investigated human cell line MDA-MB-231. An additional aim was, under the same experimental conditions, to compare the effect of the inhibitor with the effects of the existing paclitaxel drug as well as the combination of the inhibitor and paclitaxel. Methods:The newly synthesized phospholipase C inhibitor, 3-amino-5-oxo-N-naphtyl-5,6,7,8-tetrahydrothieno[2,3-b]quinoline-2-carboxamide, was administered to MDA-MB-231 triple-negative breast cancer cell line. After cell treatment with 2 M phospholipase C inhibitor and / or with 40 nM concentration of paclitaxel for 48 hours, fluorescences of immunostained samples with antibodies to CD44 and CD24 were measured by flow cytometry. Results: The inhibitor slightly but statistically significantly reduced the percentage of CSC (from 99,09±0,53 to 96,78±1,03). Paclitaxel caused a statistically insignificant increase in the percentage of CSCs. The combined treatment with inhibitor and paclitaxel did not affect the percentage of CSC. Conclusion: The newly synthesized thieno [2,3-b] pyridine reduces the percentage of stem cells in the MDA-MB-231 cell line of triple-negative breast cancer which could represent a new therapeutic approach to TNBC. The combined treatment with PLC inhibitor and paclitaxel did not affect the percentage of CSC. Therefore, it is possible that the opposite effects of paclitaxel and inhibitors cancel each other out.