In this study, we conducted whole effluent toxicity (WET) proficiency testing based on the results which EC50 value of 3 types (A, B, C) unknown samples calculated from 32 water quality measurement agencies in Korea. WET proficiency testing was expected to their improve of analysis skill and ensure reliability of analysis results. Ultimately, it is intended to promote the reliable enforcement of WET. WET proficiency testing was evaluated using the z-score, robust z-score and the results showed that 30 participating agencies were “compliance”. In addition, EC50 values of “unknown sample A” were the normal distribution. Therefore, “unknown sample A” was considered as the most suitable standard toxicity substance.