Several mutagenic and carcinogenic organic compounds have been detected in Dutch surface waters and in drinking water prepared from these surface waters. Although the levels of these compounds in drinking- and surface water are relatively low, in general below μg per litre, it appeared that organic concentrates tested in the Ames/microsome assay, showed mutagenic activity in 50 ml surface- and 500 ml drinking water. Such a result however was not expected based on the concentration of organic mutagens identified in these waters. Therefore the conclusion had been drawn that a number of unknown organic mutagens in combination with the identified mutagens were responsable for the level of mutagenic activity. With this in mind, an extensive investigation was carried out in an attempt to answer the following questions : - do drinking waters prepared from surface water, groundwater or a mixture of both show mutagenic activity; - do different water treatment processes during the preparation of drinking water influence the mutagenic activity; - what kind of physical-chemical properties have the organic compounds in drinking water concentrates showing mutagenic activity in the Ames test; - do mutagenic organic concentrates prepared from drinking water show carcinogenic properties? Against this background an inventory study was made on the presence of mutagenic activity in drinking water of 18 cities in The Netherlands. Besides this study, the influence of different water treatment processes on the mutagenic activity was examined in a number of water works. Furthermore, an attempt was made to characterize the organic compounds which are responsable for the mutagenic activity. Finally, a carcinogenicity study was carried out to see whether mutagenic drinking water concentrates induce carcinogenic effects in rats. Chapter 1, describes the aim of this investigation and the factors which have led to the present concern regarding the possible toxic effects of organic (micro)pollutants in drinking water in The Netherlands. ...