In the MSc thesis we studied the effects and activity of menadione as an inductor of oxidative stress at cellular level using yeast Saccharomyces cerevisiae as a model organism and the effects of antioxidant epigallocatechin-3-gallate (EGCG) on cells and its role in protecting cells against oxidative stress. The cells where treated with the selected compounds in different concentrations - EGCG was present in cell suspension in the concentration of 0,1 g/L and menadione in the concentration of 60 mmol/L. The effects of oxidative stress on physiological state of cells was studied using two different approaches. In the first approach, the cells were initially treated with EGCG and after that with menadione. In the second manner, the cells were treated with menadione only. Methods used for detecting such stress are various, however we used the following: determining intracellular oxidation, oxidative damage to lipids and cell metabolic energy; CFU assay and analysis of ATR-FTIR spectra. We determined that intracellular oxidation and oxidative damage to lipids in cells first treated with EGCG and then menadione is noticeably lower than in cells treated only with menadione. Cell metabolic energy in cells undergoing oxidative stress is higher due to activation of repair mechanisms. At the same time, we noticed that EGCG increases the metabolic activity of cells. The addition of EGCG did not significantly improve culturability of cells, which decreased due to oxidative stress. Additionally, changes in physiological state of treated cells were confirmed by ATR-FTIR. The differences in spectra between differently treated samples resulted mainly because of changes in lipid and carbohydrate content. Based on all the results we assess, that ATR-FTIR is a suitable method for estimating changes in intracellular redox state after treating cells with an oxidant/antioxidant.