N-cyclohexylthiophthalimide, commercial name Duslin P, at concentrations 0.5-2 micrograms/ml inhibited proliferation of V79 cells and at concentrations > 2 micrograms/ml acted cytotoxically. Inhibition of cumulative DNA synthesis correlated well with the deleterious effects of Duslin P on growth activity and plating efficiency. DNA synthesis was not renewed even 6 h after the treatment of cells. Alkaline elution of DNA of V79 cells treated with Duslin P did not confirm our expectation that this chemical compound has a DNA-damaging effect. Duslin P strongly inhibited protein synthesis at concentrations > 2 micrograms/ml. We suggest that the cytotoxic effects of Duslin P are not accompanied by any genotoxic effects.