McCoy-Plovdiv cells represent a serum-free in vitro system, which is suitable not only for diagnostic purposes but also for studies on the biological activity of different substances. In addition, it can be used for developing an expression system for production and purification of various proteins. Isolation of successfully transfected cells is an important step in this process and different selection systems can be employed for this purpose. In order to assess the potential of the antibiotic G-418 as a selective agent for transfection procedures of McCoy-Plovdiv cells, we determined its cytotoxic effect on untransfected cells. Two commercially available versions of the antibiotic were tested: products of Sigma-Aldrich and Duchefa Biochemie. Cells were treated with a wide range of G-418 concentrations and cell vitality was measured with the WST-1 reagent after 48 and 96 hours of treatment. Our results showed that concentrations in the range 0.5-0.6 mg/ml of G-418 from Sigma-Aldrich efficiently kill untransfected McCoy-Plovdiv cells whereas G-418 from Duchefa Biochemie should be applied at higher concentrations: 0.9-1 mg/ml. Furthermore, medium replacement during treatment led to higher effective antibiotic concentrations and thus to increased cell death rate, exclusively in cultures treated with high G-418 concentrations.