Groundwaters in the Bitterfeld area with its extensive mining and chemical production history are heavily contaminated with organic substances. A toxicological assessment of this pollution has to cope with three problems: (i) any substance inventory will be fragmentary and additional substances at low concentrations or transformation products may bear heavy toxicological impact; (ii) toxicity data for identified compounds are incomplete in the qualitative (spectrum of possible effects) and quantitative (lowest effect concentrations) sense; (iii) combined effects resulting from exposure to mixtures and modifying effects deriving from the specific milieu conditions are hard to predict.