In order to determine Special Environmental Quality Criteria for the Northsea and Waddensea Maximal Tolerable Risk Levels (MTR) were derived based on the risk for secondary poisoning. Therefore a model was used developed by the National Institute of Public Health and the Environment on risk-assessment for fish-eating birds. For alpha-HCH and pentachlorobenzene no toxicity data on birds could be found which made it impossible to use the model. MTR values of 28, 16, 2.9, and 1.3 ng/l were calculated for hexachlorobenzene, DDT, DDE and DDD, respectively.<br>