The primary aim of the present investigation was to study possible adverse effects of groundwater from an aquifer south of Austria's capital, Vienna, and to relate these toxicological effects to routinely measured physical/chemical parameters. Fourty-three water samples were tested for genotoxic and ecotoxic effects. For genotoxicity testing the Salmonella/microsome assay, the micronucleus test with primary rat hepatocytes and micronucleus tests with plants (Tradescantia, Vicia faba) were used. In ecotoxicity tests, algae (Selenastrum capricornutum), water cress (Lepidium sativum), and water flea (Dapnia magna) were studied as target organisms. In genotoxicity assays, 10 samples (23%) gave a weak positive response with a single end point, but only one sample (2%) was genotoxic in three different test systems. Thirty-six samples (86%) caused adverse effects in ecotoxicity assays. Plants (algae and water cress) were more sensitive than daphnie. No correlations between toxic effects and physical/chemical parameters were detected. The genotoxicity experiments indicate presently a low risk from genotoxic compounds. The ecotoxic (especially phytotoxic) properties of many water samples raise concern about their suitability for irrigation purposes. The lacking correlation between results from toxicity tests and physical/chemical data indicates that it is presently impossible to predict toxic properties from routine physical/chemical measurements with a sufficient level of safety. It is therefore important to include biological toxicity assays in groundwater monitoring programs.