Breast cancer is the most common malignant disease among women. Approximately 2500 women are diagnosed annually in the Republic of Croatia and more than 900 of them die. Breast cancer therapy depends on the stage and the molecular type of the disease. Major challenge is a triple negative breast cancer.Except for conventional options including surgical treatment, radiotherapy or chemotherapy,there is no specific treatment target. MDA-MB-231 is a breast cancer cell line with negative expression of estrogen, progesterone and HER2 (human epidermal growth factor 2) receptors. It is characterized by its aggressive nature,high degree of proliferation and rapid growth with a high tendency of metastasis resulting in a poor prognosis. According to the latest findings, interest in the context of prevention and antitumour therapy is directed to natural flavonoid compounds. Bioflavonoids are polyphenolic compounds with positive effects on human health. Quercetin is bioflavonoid with a wide spectrum of biological and pharmacological activities. In addition to significant anti-inflammatory and antioxidant activity, quercetin has anticarcinogenic and antiproliferative potential. It is proven to slow down tumour growth and progression of various types of cancer. Tumor cells reprogram energy metabolism and consume large amounts of glucose by overexpressing GLUT transporter for glucose uptake into the cell. Quercetin acts as an inhibitor of GLUT1 transporter, thus reducing the ability of malignant cells to utilize glucose by glycolysis which affects invasiveness. Quercetin also has proapoptotic effect on malignant breast cancer cells. The aim of this experiment was to investigate the effect of quercetin on the viability or metabolic activity, morphology and apoptosis of MDA-MB-231 breast cancer cells at different bioenergy conditions defined by glucose concentrations (0, 1, 5, 10, 25 and 50 mM). MTT assay was performed and the results showed that at all glucose concentrations, quercetin caused a decrease in tumor cell viability after 24-hour and 48-hour exposure.After staining MDA-MB-231 cell preparations with hematoxylin and eosin and microscopy, quercetin has been shown to affect number and morphological characteristics of cells. The Apoptosis detection was performed by TUNEL method (Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) with fluorescence microscopy analysis. However, in these bioenergy conditions and the tested concentration of 100 mM quercetin, apoptosis was not detected. Based on the preliminary results, the cytotoxic and anti-tumor activity of the bioflavonoid quercetin on the MDA-MB-231 breast cancer cells is observed and this experiment can serve as a basis for further experiments of quercetin mechanisms on tumor cells.