The plant C. majus (Celandine) is a known source of biologically active compounds that are used in traditional medicine for preventing or curing cancer diseases. The growing interest in this plant is due to discovery of a number of plant-derived cytostatics and anti-blastic drugs. The goal of our study was to discover the influence of a water extract from these plants on cancer cells and to characterize the obtained extract using different analytical methods. Water extract of leaves of C. majus were obtained and lyophilized on TelStar LyoQuest (Spain). Composition of the extract was characterized using HPLC Agilent 1100, SDS-PAGE electrophoresis and MLADI-TOF Mass-spectrometry. ADP-induced platelet aggregation in the presence of obtained mixture was studied by aggregometry using Solar2110 (Belorussia). Effects of the extracts on proliferation of cancer cells was estimated using MTT-test. Investigation of C. majus extract demonstrated presence of chelerythrine and precursors of berberine alkaloids in it. Addition of C. majus extract to platelet rich plasma induced moderate platelet activation (up to 10%). This aggregation was reversible. Further ADP-induced activation of platelets pre-incubated with the extract was diminished on 30%. The extract of C. majus leaves also was shown to possess anti-proliferative agent that was demonstrated on MCF-7 cancer cells. The surviving of MCF-7 coulter cells was decreased to 40% in the presence of extract. In conclusion, direct action of C. majus extract components on platelet aggregation and cancer cell proliferation was observed. Further fractionation for the determination of functionally active components of C. majus extract is promising for biotechnology and medicine.