The duckweed “Lemna minor” is one of the most commonly used aquatic plants in toxicity testing procedures for testing various inorganic, organic chemicals and their mixtures in aqueous solutions. The already conducted research projects on the suitability of the Lemna minor test for paper mills effluent have reported a poor reproducibility of the test results with the same sample, even in the same laboratory. Moreover, untraceable elevated toxicity values of effluent samples from graphic paper mills with deinking processes resulted within these projects. The aim of this thesis is to investigate the main parameters influencing the result of Lemna minor test. Subsequently, the suitability of the “Lemna minor test” on the phytotoxic effect of paper mills effluent can be evaluated on this basis. For this purpose, the standardized Lemna minor test including sample preparation, conservation conditions and test procedure is defined. Followed by multiple screening in seven paper mills to investigate the influence of the production process, wastewater treatment technique, and wastewater parameters on Lemna minor test results. The repeatability of the test results is investigated by three times repeating the test under standardized conditions for clarified effluent of seven paper mills in the same laboratory. Moreover, the possibility to reduce the Lemna minor test duration is conducted within this study. Based on obtained results within the standardization of the Lemna minor test, the clarified effluent samples from paper mills investigated by Lemna minor test should either be analyzed directly or be frozen after sampling. It is possible to store frozen clarified effluent samples up to two weeks prior to testing. Following the obtained results, the execution of the Lemna minor test using four dilution levels is recommended (D1, D2, D4 and D8). Prior to testing, sample filtration must be conducted via a black ribbon paper filter, and their pH is adjusted to 5.5. As a result of the test, Gw values are calculated, which reflect the first nominal dilution level (highest concentration of effluent sample) at which growth inhibition does not exceed 10 %. The reduction of growth inhibition along the wastewater treatment process could be observed for all investigated paper mills, which corresponds to the effectiveness of current wastewater treatment in paper mills. The untreated and partially treated wastewater samples are both found to have higher COD, AOX, and turbidity values. In correlation with higher contaminations, these samples showed increased growth inhibitions of Lemna minor. In this study, it could not be determined whether one of these factors or an interaction of several factors accounted for the higher growth inhibition. In the course of repeatability investigations, six of seven tested clarified effluent samples showed 1 ≤ Gw ≤ 2 based on the frond area and Gw value of 1 based on frond number. This can prove the harmlessness of the investigated clarified effluent samples and the repeatability of the test results under standardized conditions. Within the investigation of the reduction of the Lemna minor test duration, the same Gw values are obtained after five and seven days of the experiment. Due to the low toxic effect of investigated paper mills clarified effluent on Lemna minor almost the same growth inhibition can result after five days. However, this finding is only valid for clarified effluent samples of the investigated paper mills. The samples with higher toxicity can have different results.