Advanced oxidation processes are methods of oxidation, in which a very strong oxidazing agent, hydroxyl radical, is generated. They are mainly used for the oxidation of organic pollutants, which can be completly mineralized to carbon dioxide, water and inorganic compounds. When mineralization is incomplete, oxidation by-products are obtained and may be even more toxic than the initial organic molecule. Their impact on the environment needs to be assessed. The aim of this diploma thesis was to study the toxicity of ozonated wastewater. A solution of the antibiotic, tiamulin fumarate, in a concentration 400 mg/L was used as the model wastewater. The solution was treated with four ozonation cycles in a batch reactor. Each cycle lasted 10 minutes. The effects of antibiotic were determined by ecotoxicological tests on duckweed Lemna minor. In addition to toxicity testing, we also analysed the content of total organic carbon during ozonation. The results of the experiment have shown that the solution of timulin fumarate and the ozonated samples of this solution inhibit the growth of a Lemna minor and reduce the concentration of chlorophyll in fronds of the plant. The toxicity of the solution did not change after ozonation, as the differences in the specific growth of all of the ozonated samples compared to the initial antibiotic solution are very small. Despite the fact that the total organic carbon decreased during ozonation, the toxicity did not change, indicating the presence of toxic by-products.