The aim of the study was to determine the effect of phospholipase C inhibitors from the thieno [2,3-b] pyridine group on the percentage of epithelial mesenchymal CD44+CD24+, epithelial CD44-CD24+ and luminal epithelial CD44-CD24-cells positive for GM3(NeuAc) glycoconjugate and the average expression of GM3(NeuAc) per cell for each of these subpopulations of triple-negative breast cancer.Cells of the MDA-MB-231 line were cultured for 48 h in four ways: without the addition of inhibitors and/or paclitaxel, and with the addition of the inhibitor alone, paclitaxel alone, or a combination with both of these. Flow fluorescence of antibody-stained cell suspensions to CD44, CD24 and GM3(NeuAc) was measured by flow cytometry and the percentage of each cell subpopulation as well as the expression of GM3 (NeuAc) were analyzed.The phospholipase inhibitor led to an increase in the percentage and expression of GM3(NeuAc) in CD44+CD24+ and the percentage in the CD44-CD24-cell subpopulation. Only in the CD44+CD24+ subpopulation, an increased average expression of GM3(NeuAc) per cell was observed. Combination treatment with paclitaxel increased the percentage of GM3(NeuAc) even more effectively in all subpopulations.GM3(NeuAc) glycoconjugate belongs to the class of acidic glycosphingolipids and has been described as an inhibitor of signal transduction via fibroblast growth factor receptors. Because the phospholipase inhibitor from this study leads to an increase in the percentage and expression of GM3(NeuAc) in certain subpopulations, it deserves further in vitro and in vivo studies as a possible antitumor agent.