(PTPs) resulting from direct photolysis of the known aquatic micropollutants Atenolol (ATL), Metoprolol (MTL) and Propranolol (PPL) was preliminarily assessed. The standardized and commercially available luminescent bacteria test according to ISO 11348 (e.g., Microtox®, LUMIStox ®) has been frequently applied for the analysis of complex reaction mixtures resulting from direct photolysis or advanced oxidation processes (AOPs). Unfortunately, its application can lead to an underestimation of the potential environmental risk, as the standard method is not suitable for the assessment of chronic effects. To overcome this limitation, a modified luminescent bacteria test (modified LBT) for the combined analysis of acute and chronic effects towards the luminescent bacterium Vibrio fischeri was developed. Phototransformation experiments with ATL, MTL and PPL were carried out at different concentrations (100 mg/L – 400 mg/L) using a 150 W medium-pressure Hg-lamp. Primary elimination of the parent compound and mineralization of carbon during UV irradiation was monitored by HPLC-UV and dissolved organic carbon (DOC) analysis, respectively. For the toxicity testing of PTPs, photolytic mixtures were collected within 256 min of irradiation and subsequently analyzed with the modified LBT at different dilutions. After 256 min of UV-treatment, ATL, MTL and PPL were completely primarily eliminated but not mineralized according to HPLC and DOC analysis, which indicated the formation of PTPs to a high extent. In the modified LBT, a distinct increase of acute and chronic toxicity because of direct photolysis was observed for all the three beta-blockers. In case of MTL and PPL, the toxicity steadily increased with the irradiation time. In contrast, PPL showed a toxification maximum already after 8 min of irradiation. For the first time, the modified LBT was successfully applied to the toxicity testing of PTPs, revealing a high toxic potential of the investigated PTPs compared to the parent compounds. ; In this study, the environmental toxicity of phototransformation products (PTPs) resulting from direct photolysis of the known aquatic micropollutants Atenolol (ATL), Metoprolol (MTL) and Propranolol (PPL) was preliminarily assessed. The standardized and commercially available luminescent bacteria test according to ISO 11348 (e.g. Microtox®, LUMIStox®) has been frequently applied for the analysis of complex reaction mixtures resulting from direct photolysis or advanced oxidation processes (AOPs). Unfortunately, its application can lead to an underestimation of the potential environmental risk, as the standard method is not suitable for the assessment of chronic effects. To overcome this limitation, a modified luminescent bacteria test (modified LBT) for the combined analysis of acute and chronic effects towards the luminescent bacterium Vibrio fischeri was developed. Phototransformation experiments with ATL, MTL and PPL were carried out at different concentrations (100 mg/L – 400 mg/L) using a 150 W medium-pressure Hg-lamp. Primary elimination of the parent compound and mineralization of carbon during UV irradiation was monitored by HPLC-UV and dissolved organic carbon (DOC) analysis, respectively. For the toxicity testing of PTPs, photolytic mixtures were collected within 256 min of irradiation and subsequently analyzed with the modified LBT at different dilutions. After 256 min of UV-treatment, ATL, MTL and PPL were completely primarily eliminated but not mineralized according to HPLC and DOC analysis, which indicated the formation of PTPs to a high extent. In the modified LBT, a distinct increase of acute and chronic toxicity because of direct photolysis was observed for all the three beta-blockers. In case of MTL and PPL, the toxicity steadily increased with the irradiation time. In contrast, PPL showed a toxification maximum already after 8 min of irradiation. For the first time, the modified LBT was successfully applied to the toxicity testing of PTPs, revealing a high toxic potential of the investigated PTPs compared to the parent compounds. Acknowledgements: This study was funded by the German Federal Ministry of Education and Research, funding code 03X0094C. Jakob Menz received a scholarship for part of this work from the Innovations-Inkubator Lüneburg (Teilmaßnahme 1.4 Graduate School).