Cancers are a set of different diseases that differ from each other in their etiology, biology and clinical picture, and consequently in the method of treatment. Cancer is most often clinically manifested as a cluster of a large number of malignant transformed cells whose number is progressively increasing due to uncontrolled cell division. Cancer is the second leading cause of death in the world, and is responsible for 9.6 million of deaths in 2018. Bladder cancer is the 9th most common form of malignancy and the 13th most common cause of death from cancer worldwide. Breast cancer is the most common tumor in women in the developed countries of the world. In Croatia, it is also in first place in terms of incidence, because about 2,600 women get sick in Croatia every year. The aim of this thesis is to examine the cytotoxic effect of capers (Capparis spinosa L.) on human breast cancer cell line (MDA-MB-231) and bladder cancer cell line (T24). It is assumed that the number of cancer cells will decrease, consequently by exposing the cells to capers extract obtained by different isolation methods, relative to the control group. The MTT test was performed by a cytotoxic assay on MDA-MB-231 and T24 cell lines. The survival rate of cancer cells was obtained by comparing the absorbance of formazan in cells treated with C. spinosa extract obtained by different methods of isolation and the absorbance of untreated cells. Incubation times for the cytotoxicity test were 4, 24, 48, and 72 hours. The presentation of the results is graphical where the dependence of the incubation time and the percentage of metabolically active cells is shown. The cytotoxic effect of capers on T24 bladder cancer cells is significant at concentrations of 50 µg/mL and 100 µg/mL. The greatest cytotoxic effect was exerted by a capers from Brač whose extract was obtained by microwave distillation at a concentration of 50 µg/mL during an incubation time of 72 hours. The most significant cytotoxic effects of caper extract on breast cancer were seen at higher concentrations of 50 µg/mL and 100 µg/mL. The most significant effect on MDA-MB-231 cell line was achieved with a caper from Brač whose extract was obtained by microwave extraction at a concentration of 10 µg/mL during the 72h incubation period. In vitro exposure of breast cancer cells (MDA-MB-231) and bladder cancer cells (T24) to caper extracts (Capparis spinosa L.) isolated by various isolation methods leads to reduced cancer cell survival. The tested caper extracts achieve a cytotoxic effect that depends on the concentration and incubation time. The cytotoxic effect of caper (Capparis spinosa L.) has been confirmed, which is also the hypothesis of this paper. Since there is no such study, i.e. the cytotoxic effect of capers on MDA-MB-231 and T24 cell lines, it would be good to repeat the experiment to confirm the results and, if necessary, upgrade the obtained results. This research has been fully supported by the Croatian Science Foundation under the project (HRZZ-IP-06-2016-1316 "Plants as a source of bioactive sulphur compounds and their ability to hyperaccumulate metals").