Objective — To investigate the apoptotic activity of Gratiola officinalis L. extract on human tumor cell lines by flow cytofluorometry. Material and Methods — The extract of Gratiola officinalis L. was manufactured via our original methodology. Studies were performed on human tumor cell lines: HeLa – cervical carcinoma, Jurkat – T-cell lymphoblastic leukemia, MCF-7 – breast adenocarcinoma, A549 – lung carcinoma, PC-3 – prostate carcinoma, HCT-116 – colon carcinoma, A498 – renal carcinoma, and SK-BR-3 – human breast carcinoma. Induction of apoptosis was studied after incubating cell lines with Gratiola officinalis L. extract at a concentration of 0.9 mg/mL using the Annexin V-FITC Apoptosis Kit. Caspase-dependent apoptosis was examined on a flow cytometer using anti-caspase-3-FITC (BD) kit on the Jurkat cell line. Morphological studies of HeLa cervical carcinoma cells in the alive and dead test were performed using two stains, acridine orange and propidium iodide, at different concentrations of the extract. The statistical data processing was performed using Microsoft Office Excel software. Results — One day after their exposure to Gratiola officinalis L. extract at a concentration of 0.9 mg/mL, tumor cells were mostly in late apoptosis stage. Cytotoxic activity of Gratiola officinalis L. extract was established for all investigated tumor cell cultures but their sensitivities to the extract were different. Mechanisms of antitumor action of Gratiola officinalis L. extract were identified: we established that the extract induced caspase-dependent apoptosis in tumor cells. Conclusion — The identified mechanisms of apoptotic activity of Gratiola officinalis L. extract confirmed the prospects of bioflavonoids as new-generation antitumor agents.