Macrocyclic compunds, especially crown ethers are of great importance in chemistry and biology mostly due to their ability to selectively bind cations. Crown compounds possess ionophoretic properties, but their antitumor activity has been underexplored. In this diploma thesis we studied the effect of 9 macrocyclic compounds on proliferation of human tumor cells HeLa, SW620, MCF-7, H460 and MiaPaCa-2 and evaluated the mechanism of action of the most active compounds. We used MTT assay to assess the proliferation rate of tumor cells. Compounds LET-610, LET-612, LET-646 and LET-613 showed significant cytostatic and/or cytotoxic activity. For some compounds it was previously demonstrated that the size of hydrophilic cavity, type of donor atoms and groups that penetrate into the cavity affect ion binding, whence hydrophobic ring with supstituents is important for membrane insertion. We showed that these structural characteristics correlate perfectly with the antiproliferative activity in tumor cells. In order to elucidate the mechanism of action of the most active tested compound LET-613 (IC50 ≈ 2 μM), we used flow cytometry for the cell cycle analysis. LET-613 induced G1 arrest of human pancreatic cancer cell MiaPaCa-2, along with S phase reduction and subsequent increase in subG1 phase (apoptotic cells). Activation of apoptosis in human cancer cells treated with compounds LET-613 and previously tested LET-329 was confirmed by performing Annexin V test that specifically detects and quantifies apoptotic cells. The results support the hypothesis that crown compounds disrupt ion transport that leads to the cell cycle perturbations and activation of apoptosis in tumor cells.