The aquatic environment contains numerous mixtures of various combinations of hazardous single substances, and it is difficult to calculate the toxicity values of all mixtures by test methods. Therefore, the estimation method based on the UN-GHS model has been widely used. However, there are cases where the difference between the model estimate and the actual (test) value is large, indicating the need for research to reduce the uncertainty of the model. This study aims to develop a modified model that can solve the problems of the UN-GHS model, and to evaluate the applicability of the modified model through toxicity testing of mixtures. An acute Daphnia magna toxicity test was conducted according to the drug mixture exposure, and the difference between the toxicity test result and the model estimate was analyzed assuming that it was derived from the model uncertainty. The large difference between the UN-GHS model estimate and the test result was due to the toxicity weighting factor, determined by dividing the L(E)C50 of each single substance into interval values. Therefore, in this study, the existing model was improved by developing a modified model that assigns a toxicity weighting factor to the point value, rather than the interval value of L(E)C50 for each single substance. T...