The aim of the research was to demonstrate the effect of phospholipase C inhibitor, alone or in combination with paclitaxel, on reducing the percentage of epithelial CD44-CD24+ and luminal epithelial CD44-CD24- cells positive for CD15s and on lowering the average expression of CD15s glycoconjugate per CD44-CD24+ and CD44-CD24+ cell. A newly synthesized phospholipase C inhibitor from the thieno [2,3-b] pyridine group was used on MDA-MB-231 triple-negative breast cancer cell line. Cells were treated with a 2 µM phospholipase C inhibitor and / or paclitaxel for 48 hours, after which the fluorescence of antibody-stained samples on CD44, CD24 and CD15s was measured using a flow cytometer. The results are presented in statistical diagrams showing the percentages of luminal and epithelial cells of the MDA-MB-231 cell line positive for CD15s and the average expression of CD15s per cell of these population cells by groups depending on the use of the inhibitor, paclitaxel or a combination thereof. Paclitaxel decreased CD15s expression on epithelial cells without statistical differences. Inhibitor 6 increased, and paclitaxel decreased CD15s expression in luminal cells, with no statistical significance. Inhibitor 6 and paclitaxel decreased the mean expression of the CD15s marker per epithelial cell with statistical significance, p = 0.011. Paclitaxel decreased the mean expression of the CD15s marker per luminal cell without statistical significance. Treatment of MDA-MB-231 cell line with 2 µM concentration of thieno [2,3-b] pyridine phospholipase C inhibitor statistically significantly reduced the percentage of epithelial CD44-CD24 + cells per CD15s expression per breast cancer stem cell, but did not significantly change the percentage of CD44 -CD24 + subpopulations of CD15s marker-positive stem cells.