The ability of artificial ribonucleases to cause in the concentration-dependent manner death of cancer cells has been studied. The cytotoxic activity of artificial ribonucleases is observed at rather low concentration of these compounds (10(-5) M). Analysis of the mechanism of artificial ribonucleases cytotoxicity revealed that compounds under the study exhibit membranotropic activity in addition to ribonucleases activity found earlier. This activity is responsible for effective penetration of these compounds inside cells. The results obtained show that artificial ribonucleases induce cell death via damage of cells membrane, detachment of plasmalemma and derangement its macromolecular organization. In the case of short-term exposure of cells to the compounds, cells, even with damaged membrane, survive.